A method of preheating an unfinished stator as an intermediate product before a stator as a final product for a motor is provided in order to preheat a mold for molding together with the unfinished stator in short time by using preheating facilities with simplified structures. In the method, the mold has a hollow and longitudinal barrel portion and a flange portion extending outward from an end of the barrel portion. The flange portion is arranged at a horizontal position to turn the barrel portion upward and an end of a cylindrical and longitudinal frame is put on the flange portion in a state where a core of the unfinished stator is fitted in the frame. A heat-resistant box of which an underside is opened covers the frame and the mold to surrounded outsides of the frame and the mold.