An object of the present invention is to effectively cool the generation of heat of a resin molding armature, especially to effectively cool the generation of heat of a coil in an electric motor having a resin molding armature and in an electric tool into which the electric motor is incorporated. In order to effectively cool the generation of heat of a resin molding armature 2a, especially in order to effectively cool the generation of heat of a coil 2e, an open groove 2j is provided for each slot 2g on an outer circumferential face of a molding resin filling layer 9a, which is filled into the slot 2g of a core 2f, in which the coil 2e is embedded. A molding die used for forming a molding resin filling layer 9a is made up of four members which can be split in four sliding directions (in the radial direction of the core).