PURPOSE: To prevent the interruption of heating caused by, for example, the disconnection of a resistance wire heater and to facilitate a temperature fall, by a method wherein a part of a circulating system of fluid for heating a mold is formed with a magnetic metal pipe, and an electromagnetic induction heating coil is wound around the pipe to control electric current flowing in the coil.

CONSTITUTION: A heating fluid circulating system, which heats molds 1 and 2, and consists of hoses 3W9, and a pump 13, is formed at the outside of the molds 1 and 2 for plastic molding. A pipe 10, made of a magnetic metal, is installed to a part of the circulating system to form an electromagnetic induction heating part 12 in which an electromagnetic induction heating coil 11 is wound around the pipe 10. A current to the coil 11 is controlled by a drive circuit of a control panel 16.