Oshima Island, also known as Mutsu Oshima, is located in Hiranai Town, Higashitsugaru District, Aomori Prefecture. The island lies in the central part of Mutsu Bay, just offshore from Natsudomari Cape, the northern tip of the Natsudomari Peninsula.
One of the island’s greatest attractions is its easy access. A footbridge connects Oshima with the Natsudomari Peninsula, allowing visitors to walk to the island even at high tide. This makes Oshima an appealing destination for travelers who want to experience the natural beauty of a small Japanese island without taking a boat.
At the northern tip of the island stands the Mutsu Oshima Lighthouse, which has helped guide ships traveling through Mutsu Bay for many years.
The Natsudomari Peninsula extends prominently into Mutsu Bay. From its tip, visitors can cross a bridge and arrive at Oshima, where a peaceful coastal landscape awaits.
On clear days, the view from the bridge is particularly impressive, with the deep blue waters of Mutsu Bay stretching into the distance. The island’s rocky shoreline also offers an opportunity to see the natural shapes and textures created by geological processes up close.
Because of its compact scale and scenic surroundings, Oshima is a pleasant place to explore at a leisurely pace while enjoying the sea, rocks, forests, and open skies.
Oshima is located approximately 180 to 200 meters offshore from Natsudomari Cape. The island is roughly gourd-shaped or oval, with a circumference of about 3 kilometers and an area of approximately 20 hectares. Its central area consists of gently rolling terrain, reaching an elevation of around 73 meters.
Oshima was once connected to the Natsudomari Peninsula by a sandbar. According to local accounts, people could walk between the peninsula and the island at low tide. Over time, however, erosion caused the land connection to disappear, leaving the island separated from the peninsula except for the present-day bridge.
The northern part of the island is composed mainly of dacite associated with Tertiary volcanic activity. Particularly along the western coast, visitors can see steep cliffs where columnar jointing has developed in the volcanic rock. These distinctive formations create some of the island’s most striking coastal scenery.
The southern part of Oshima is dominated by shale. Geological layers containing shell fossils can be seen here, providing evidence of ancient geological changes and uplift. The southern landscape is also characterized by broad-leaved trees such as oak and stands of Japanese black pine, creating a calmer and greener environment than the rocky northern coast.
Oshima has appeared in historical records for centuries. Maps dating from the early Edo period identify the island as “Oshima” together with Natsudomari Cape. A small settlement known as Ōma Village is also said to have once existed in the surrounding area.
The residents of the former settlement are believed to have made a living through activities such as salt production. As times changed, however, the settlement was eventually abandoned.
The island is also known as “Tazawa no Baketo-jima” and is counted among the Seven Mysteries of Hiranai. According to local folklore, Oshima could appear to change in size or distance depending on the weather and time of day.
In the past, people are said to have used these changes in the island’s appearance as a clue for judging approaching weather conditions. Such stories add an intriguing sense of mystery to the island and demonstrate how closely local communities observed their natural surroundings.
At the northern tip of Oshima stands the Mutsu Oshima Lighthouse. The unmanned lighthouse began operating in 1949 and is approximately 9.7 meters tall. It emits a white single flash once every seven seconds, with its light reaching approximately 14 kilometers.
The island also has a connection with traditional maritime beliefs. During the Edo period, Benzaiten, a deity associated with protection and good fortune, was enshrined here as a prayer for safe travel at sea. The shrine continues to be worshipped today as Benten-gu, adding a spiritual dimension to the island’s natural scenery.
Today, Oshima is enjoyed as a destination where visitors can experience nature up close. The waters around the island are known as good fishing grounds, and anglers often gather around the rocky coastline below the lighthouse.
Rental boats are also available, making it possible for beginners to enjoy fishing on the surrounding waters. Oshima has a longer history as a tourist destination as well. In the 1960s, an underwater sightseeing boat with a glass-bottomed hull operated in the area, allowing visitors to observe the underwater world of Mutsu Bay.
Oshima changes its appearance with the seasons. In spring, wildflowers such as dogtooth violets bloom across the island. In summer, the vivid blue of the sea and sky creates a refreshing coastal landscape.
It is not possible to walk completely around the island, but the views from the bridge and designated scenic points provide excellent opportunities to appreciate the surrounding landscape. The combination of coastal scenery, unusual geological formations, history, local legends, and maritime traditions makes Oshima one of the highlights of a visit to the Natsudomari Peninsula.
Oshima offers a chance to encounter the natural and cultural character of northern Japan in a compact setting. Visitors can cross the bridge on foot, admire the rocky coastline and Mutsu Bay, visit the area around the lighthouse, or simply enjoy the quiet atmosphere of the island.
For travelers exploring Aomori beyond its major cities and famous attractions, Oshima is a rewarding place to discover the scenery, geology, traditions, and stories of the Mutsu Bay coast.