Tomogashima (友ヶ島) is a cluster of four islands in the Inland Sea, off Wakayama, Wakayama, Japan. The four islands are Jinoshima (地ノ島), Kamishima (神島), Okinoshima (沖ノ島), and Torajima (虎島). The islands form part of the Setonaikai National Park.^[1][2]

The islands were used by Buddhist monks for Shugendō. The folklore of the region holds that Ennogyoja, the founder of shugendo, underwent training on the steep cliffs of Tomogashima in the seventh to eighth centuries. This gave rise to the nickname of "The island of shugendo (mountain asceticism)" for Tomogashima.

Later, during the Meiji period, a brick fort and lighthouse were built.^[1][3] Also during this time gun batteries and other defences, along with various support facilities, were constructed to counter foreign warships. Tomogashima was a critical component of the Shusei Kokubō (守勢国防 - i.e. "Static Defence") policy of the 1870s and 1880s, which emphasised coastal defences. Access to the cluster by the public was strictly prohibited by the Imperial Japanese Army up to the end of World War II.^[4]

Tomogashima is the subject of a scroll of 1661 in the Shōgo-in in Kyoto by Kanō Tan'yū as well as an anonymous work of 1798 in the British Museum.^[3][5] The island became the prototype for fictional Hitogashima Island in Summer Time Rendering anime and manga series.

• Tomogashima Lighthouse
• Setonaikai National Park
• Imperial Japanese Navy
• List of Army Fortresses in Japan