To provide a constraining tool for a flat plate body and a constraining method, constraining a flat plate body such as a cooling fin of a transformer efficiently and simply.

This flat plate body constraining tool constrains the flat plate bodies 30 to each other in cutting a structure in which a plurality of flat plate bodies 30 are arranged in parallel at spaces and combined. The tool includes: a plurality of columnar bodies supporting a plurality of claw members 12 having an engagement groove 12A engaged with the side surface of the flat plate body 30 like a skewer by a shaft 14; a frame member 26 provided with a plurality of parallel slide tracks 25A slidably supporting the lower end parts of the plurality of columnar bodies 16; and a fastening member 17 connecting the plurality of columnar bodies 16 by each slide track 26A and narrowing the space between the plurality of columnar bodies 16.