A core is formed by laminating a plurality of core plates. Each of the core plates includes a joint portion for joining the core plates together in the laminated state. Each of the joint portions includes a fixing piece, a passage hole, a receiving portion, and an opening. Each of the fixing pieces is raised in the direction in which the core plates are laminated. Each of the passage holes receives the fixing piece of the corresponding adjacent one of the core plates. The fixing piece in the passage hole is bent to join the receiving portion to the adjacent core plate. Each of the openings receives the corresponding one of the fixing pieces to avoid interference with the fixing piece bent onto the receiving portion of another core plate.