PURPOSE: To detect the optimum timing for mold release and consequently prevent defective molding from developing by a method wherein the interior temperature of an expansion-molded part is measured by a temperature sensor protruded in a cavity and a mold temperature is measured by a temperature sensor attached to a mold.

CONSTITUTION: Both a temperature sensor 3 attached in such a manner as protruded in a cavity 2 and another sensor 4 attached to a mold 1 are connected to a control device 5. Firstly, raw pellets are charged in the cavity 2. Secondly, steam is supplied in the cavity 2 in order to foam-fuse said pellets. Thirdly, when the sensor 3 detects the temperature rise up to the predetermined temperature, the control unit 5 issues the command to stop the supply of steam and to supply cooling water in order to cool the mold 1. Fourthly, after the sensor 4 reads that the surface temperature of the molded part lowered down to the releasing temperature of the molding, the mold is opened and the molded part is released from the mold. Thus, the more efficient heating and cooling cycles are realized and consequently defective moldings are prevented from developing.