To reduce a cost without necessity of providing a complicated control circuit to a high frequency induction heater by a method wherein a conductive powder is added to a thermosetting resin.

A conductive powder is added by a specific mixing ratio to a thermosetting resin. After pouring that into a cavity of a molding tool 2, high frequency voltage is impressed to electrode plates 1, 1 pressurizing the molding tool with a pressurizing mechanism 6 for specific time. Thereby, the thermosetting resin is crosslinked. Since temperature of the thermosetting resin is converged to a specific value with an elapse of heating time thereby, it comes not to be required that impression and stop of the high frequency voltage are repeated. Consequently complicated control circuit is not required to be equipped to a high frequency induction heater, and a cost of equipment can be significantly reduced.