In 1867 a steamship called the Iroha Maru, which was led by the famed political activist Sakamoto Ryōma and the association of lordless samurai called the Kaientai, collided with the Meikō Maru, a warship of the Kishū Domain off the coast of Tomonoura,. The Iroha Maru and its cargo sank, which brought Ryōma and others to Tomonoura, where they stayed for several days to negotiate repayment for the damage incurred.

This museum exhibits the remains of the Iroha Maru recovered through five salvage missions beneath the sea. It also displays illustrations and photographs related to the incident. Visitors can also see a diorama reproducing the sinking of the Iroha Maru.

The second floor of the museum contains a recreation of a secret room in the house of a merchant named Masuya Seiemon. This room is where Ryōma and members of the Kaitentai hid from their enemies during their stay in Tomonoura.