To provide a tool for cutting a fish into fillets, which can clearly cut the fish into the fillets without needing a skill.

This tool 10 for cutting the fish into the fillets comprises a piano wire 12 disposed in an approximately U-shape and a piano wire-supporting member 16 which supports both the ends of the piano wire 12 and forms an opening 14 for cutting the fish together with the piano wire 12. For example, the tip portion 12a of the piano wire 12 is pressed against the gill 2b portion of a sardine 2 and then pulled toward the caudal fin, thereby cutting off the fish body into fish halves, respectively. When a thin belt plate such as a conventional tool for cutting a fish into fillets is used, it is necessary to press the belt plate against the body of the fish at a prescribed angle. Even when the piano wire 12 which is a wire and has high rigidity is used at an arbitrary angle to a fish body, the piano wire 12 can easily be reached to the position of the midrib and then moved along the midrib without allowing the piano wire 12 to dig into the midrib.