A slot insulating paper (12) is arranged in a slot (18) of a core (10) of an electric motor or generator. In a state where the slot insulating paper (12) is spread in a sheet shape, a region (high stiffness portion (36)) that is part of the slot insulating paper (12) is a breakage resistance portion having a higher resistance against breakage than the other region (ordinary portion (38)). The slot insulating paper (12) may be formed by laminating a plurality of insulating layers. In the breakage resistance portion, for example, at least one of the laminated insulating layers is a high stiffness layer formed of a high stiffness material. Thus, the breakage resistance portion is formed to have a high stiffness. With a high stiffness, it is possible to improve breakage resistance.