In order to prevent a stamp plate or a stencil plate from falling off or shifting on. a stamp unit, and to simplify operations for exchanging the stamp plate (stencil plate) during stamping operations or stencil-producing operations, a support body is formed with rail portions. The rail portions are C-shaped in cross section. Holding pawl portions are formed along lengthwise edges of the rail portions. The stamp plate is held in a recess of a backside reinforcement. When the stamp plate held in this manner is inserted in a space between the pair of rail portions, the holding pawl portions hold the lengthwise edge surfaces of a stamp surface of the stamp plate, and also the rail portions support the backside reinforcement so the stamp plate will not fall off.