To prevent the breakage of induction coils by an electromagnetic mechanical reaction force, and make the installation simple and inexpensive.

A mold 10 is heated by giving an alternating current of substantial sine waves to induction heating coils 14a, 14b, 14c, (14) provided on a molding surface 11 and a rear surface to be an opposite side of the metallic mold 10 from a transistor inverter 20a. The output of a transistor inverter 20a is controlled by subcontrollers 21a, 21b, 21c so that temperatures detected by temperature sensors 25a, 25b, 25c provided on a part of the mold 10 to be heated become predetermined temperatures. In the case of providing a plurality of induction coils 14, each magnet switch 22a, 22b, 22c set between each induction coil and one transistor inverter 20a is opened and closed by each subcontroller, thereby controlling heating temperature every induction coil 14.