Abstract: Disclosed is a reinforcement structure of a rectangular flat metal plate, which is provided with: a rectangular flat metal plate that is predominantly subjected to in-plane shear, and supports a compressive load as necessary; strip-like rectangular section members that are spliced in parallel with both side edges of the flat plate in the longitudinal direction so as to reinforce the flat plate; and a plurality of square tube-like members that are parallelly arranged for each constant interval in the shorter side direction of the flat plate, and are spliced on one side surface of the flat plate, or are spliced so as to overlap one another across the flat plate between both surfaces of the front and back of the flat plate, wherein the torsional rigidity and torsional strength of the rectangular flat metal plate are increased to ensure a yield shear load, and shear yield strength can be stably maintained even in the transition of shear deformation after the yield.