Tomogashima Island is a group of uninhabited islands located in the Kitan Strait off Kada, northwest of Wakayama City, Wakayama Prefecture. The island group consists of four islands: Okinoshima, Jinoshima, Torajima, and Kamijima.
Today, Tomogashima is known for its rich natural environment as part of the Setonaikai National Park. However, the islands also have a fascinating military history. From the Meiji period through the end of World War II, Tomogashima played an important role as part of a major coastal defense system.
Scattered throughout the island are the remains of artillery batteries, brick tunnels, ammunition storage areas, observation posts, and other military structures. Surrounded by dense forest, these quiet ruins offer visitors a unique opportunity to experience the intersection of modern Japanese military history and nature.
The Yura Fortress was a large-scale military fortress system constructed by the Imperial Japanese Army to protect Osaka Bay. Together with the Tokyo Bay Fortress and Shimonoseki Fortress, it was classified as a First-Class Fortress, reflecting its strategic importance.
Construction began in 1889, during the Meiji period. The fortress system extended across a wide area around the Kitan Strait, including Awaji Island, Tomogashima, Kada, and Miyama on the Wakayama side.
At the time, Osaka Bay was an important center of Japan’s economy, industry, and politics. Protecting the entrance to the bay was therefore regarded as a matter of national importance.
Because Tomogashima sits at a strategic point in the Kitan Strait, the islands became a front-line defensive position controlling maritime traffic entering and leaving Osaka Bay.
The Yura Fortress was divided into several defensive areas. The Tomogashima area was one of its principal strongholds.
Artillery batteries numbered First through Fifth were established mainly on Okinoshima and Torajima. Together with the fortifications in the Yura area on Awaji Island, they formed a coordinated defense system designed to attack enemy warships attempting to pass through the strait.
The batteries were broadly divided into long-range artillery batteries and interception batteries.
Long-range batteries were generally built in elevated areas so that enemy ships could be attacked from a distance. Their artillery included howitzers, with fire directed using information from observation posts.
Interception batteries, on the other hand, were positioned to fire at enemy vessels passing through the narrow strait from the side. These batteries used cannons with relatively flat trajectories and were designed to create a powerful crossfire with batteries on the opposite side.
The First Battery was constructed on cliffs at the western end of Tomogashima. Its position allowed it to directly target ships passing through the Kitan Strait.
Today, the ruins are located next to Tomogashima Lighthouse. From this elevated location, visitors can enjoy wide views toward Awaji Island and Osaka Bay.
Remains of observation posts can still be seen on both sides of the battery. These structures provide an impression of how information and commands were communicated during the fortress’s operational period.
The observation facilities are particularly valuable because relatively substantial portions survived the postwar period, making them important surviving examples of the island’s military heritage.
The Second Battery was an interception battery built near the coast. It was designed to work together with batteries on the Awaji Island side and attack enemy ships caught between the two defensive positions.
After the end of World War II, the facility was deliberately demolished. Subsequent typhoons and the forces of nature caused further deterioration, leaving sections of the concrete structure dramatically collapsed.
The ruins are currently off-limits for safety reasons, but they can be viewed from outside. The broken concrete walls and rugged landscape create a striking scene where visitors can observe both the physical traces of war and the powerful effects of nature over time.
The Third Battery was the largest artillery battery on Tomogashima and served as one of its principal defensive positions. It was built near the highest part of the island and featured a distinctive circular layout designed to provide a 360-degree field of fire.
Underground facilities included ammunition storage areas and equipment for moving shells to the gun positions. Portions of these underground structures can still be explored today.
The combination of red brick passageways, underground chambers, moss-covered walls, and surrounding forest creates an atmospheric landscape unlike many other historic sites in Japan.
The ruins have also become popular among photographers and filmmakers because of their mysterious, almost otherworldly atmosphere. In recent years, some areas have additionally been used as an “Art Museum of Sound,” creating an unusual experience in which sound and light interact with the historic underground spaces.
The Fourth Battery served a supporting role to the Third Battery and was designed to defend the waters to the east.
Its basic structure was similar to that of the Third Battery, but on a smaller scale, and it was also designed to allow firing in multiple directions.
Because it was decommissioned relatively early, the Fourth Battery escaped some of the destruction that affected other military facilities after the war. As a result, it remains a valuable surviving example of the original fortress design.
The Fifth Battery was established to defend areas that could not be adequately covered by the other batteries, including the central part of the island.
Important facilities such as barracks, a power station, and ammunition storage areas were concentrated in the interior, making protection of this area essential to the overall defense of Tomogashima.
Although smaller than some of the other batteries, the Fifth Battery played an important role in the island’s internal defense system.
Tomogashima Lighthouse is another important landmark on the island. It was built in the early Meiji period by British engineer Richard Henry Brunton, who was involved in the construction of many of Japan’s early Western-style lighthouses.
The lighthouse has a history connected with the development of the fortress, including the relocation of its position as military defenses were constructed. Its presence illustrates how Tomogashima was important not only for military defense but also for maritime navigation.
The lighthouse continues to help guide ships through the surrounding waters and is also recognized as a Registered Tangible Cultural Property.
The Yura Fortress was built as a major national defense project, but the nature of warfare changed dramatically with the development of aviation.
By World War II, air power had become increasingly important, and the fortress did not experience the major naval battle for which it had originally been designed. Japan’s surrender brought an end to its military role.
After the war, Tomogashima eventually became part of the Setonaikai National Park, while many of the former military facilities came to be valued as historical heritage.
Today, visitors can explore Tomogashima on foot while discovering the remains of its former military infrastructure. Hiking trails lead through forests and along coastal landscapes, revealing artillery positions and brick structures along the way.
The contrast between the quiet forest, rugged coastline, weathered brick ruins, and traces of military engineering gives Tomogashima a distinctive atmosphere.
For photographers and history enthusiasts, the island offers countless opportunities to observe how human structures have gradually become part of the natural landscape. The moss-covered walls and trees growing around the ruins create scenery that feels far removed from modern city life.
The ruins of the Yura Fortress are much more than abandoned military buildings. They preserve important evidence of Japan’s modernization, coastal defense strategy, and military history during the Meiji period and the first half of the 20th century.
At the same time, Tomogashima offers beautiful forests, coastal scenery, and panoramic views across the Kitan Strait. This combination makes the island appealing to visitors with very different interests—from history and architecture to hiking, photography, and nature.
Walking through the ruins can also encourage reflection on the changing nature of warfare and the importance of peace. Structures once built for defense now stand quietly among trees and vegetation, allowing visitors to encounter history in a very different atmosphere from the one in which these facilities originally operated.
The Yura Fortress ruins on Tomogashima are a remarkable cultural heritage site where modern Japanese military history and the natural environment have become intertwined.
The five artillery batteries, underground passages, observation posts, historic lighthouse, and other remains reveal the strategic importance once attached to the Kitan Strait. Meanwhile, the forests and coastline surrounding these structures have transformed the former military landscape into a unique destination for exploration.
For travelers looking beyond Japan’s famous temples and castles, Tomogashima offers a different perspective on the country’s past. Here, history is not displayed behind glass—it remains quietly embedded in the landscape.
A visit to the island provides an opportunity to walk among historic ruins, enjoy the beauty of the Setonaikai National Park, and reflect on the passage of time and the value of peace.