PURPOSE: To adjust the temperature of steam in the hot steam chamber of a molding device with a simple device by arranging a pressure governor for letting compressed air flow out of the air pressure device in a reducing valve device in proportion to the steam pressure in the hot steam chamber.

CONSTITUTION: In a device for expanding raw materials by supplying hot steam to a male and a female hot steam chamber S1, S2, reducing valve devices X1, X2 which adjust the apertures of steam supplying passages Y1, Y2 through air pressure devices Z1, Z2 are arranged to each of the steam supplying passages Y1, Y2 to each hot steam chamber S1, S2. To these reducing valve devices X1, X2 are arranged respectively pressure governors C1, C2 for allowing compressed air to flow out of the air pressure devices Z1, Z2 in accordance with the steam pressure of the compressed steam chambers S1, S2. In this manner, the temperature of steam in the hot steam chamber in a plastic foam-molding device can be adjusted with a simple device.