A method of preheating an unfinished stator for a motor, as a premise that the unfinished stator is an intermediate product before a stator as a final product and the stator comprises a core in which a plurality of annular electromagnetic steel sheets are laminated and coils bound at a plurality of grooves formed at an inner peripheral portion of the core at a predetermined distance, wherein the unfinished stator is preheated before being performed molding in which an insulative resin is injected in clearances at least between the core and the coils and is cured, and wherein a mold for molding comprises a hollow and longitudinal barrel portion and a flange portion extending outward from an end in a longitudinal direction of the barrel portion, characterized in that: the method comprises the steps of: - arranging the flange portion of the mold for molding at a horizontal position to turn the barrel portion of the mold for molding upward; - putting an end of a cylindrical and longitudinal frame on the flange portion of the mold for molding in a state where the core of the unfinished stator is fitted in the frame; - covering the frame and the mold for molding with a heat-resistant box of which an underside is opened to surround outsides of the frame and the mold for molding; - turning on an alternative current to the coils in the heat-resistant box to cause the coils to generate heat; and - preheating the core, the frame and the mold for molding by the heat which is conducted from the coils. wherein the unfinished stator is preheated before being performed molding in which an insulative resin is injected in clearances at least between the core and the coils and is cured, and wherein a mold for molding comprises a hollow and longitudinal barrel portion and a flange portion extending outward from an end in a longitudinal direction of the barrel portion, characterized in that: the method comprises the steps of: - arranging the flange portion of the mold for molding at a horizontal position to turn the barrel portion of the mold for molding upward; - putting an end of a cylindrical and longitudinal frame on the flange portion of the mold for molding in a state where the core of the unfinished stator is fitted in the frame; - covering the frame and the mold for molding with a heat-resistant box of which an underside is opened to surround outsides of the frame and the mold for molding; - turning on an alternative current to the coils in the heat-resistant box to cause the coils to generate heat; and - preheating the core, the frame and the mold for molding by the heat which is conducted from the coils. - arranging the flange portion of the mold for molding at a horizontal position to turn the barrel portion of the mold for molding upward; - putting an end of a cylindrical and longitudinal frame on the flange portion of the mold for molding in a state where the core of the unfinished stator is fitted in the frame; - covering the frame and the mold for molding with a heat-resistant box of which an underside is opened to surround outsides of the frame and the mold for molding; - turning on an alternative current to the coils in the heat-resistant box to cause the coils to generate heat; and - preheating the core, the frame and the mold for molding by the heat which is conducted from the coils.