PURPOSE: To perform preheating uniformly and in a short time by making heat insulation action low by gaps among particles each, by making the title device into constitution where a powdery and granular molding material loaded into a preheating cavity through an upper opening is preheated by a heating device while compressing by an upper and lower plungers.

CONSTITUTION: A preheating pot 2 is positioned directly below a measuring device B, a measuring rotor 15 is turned, a fixed quantity of a powdery and granular molding material 1 is loaded into a cavity 4 of the preheating pot 2 and the molding material 1 is stayed within a cavity 4 by making the top of a lower plunger 5 into the bottom. Then a movable platen 22 is moved, the preheating pot 2 is moved to a position directly below an upper plunger 6 from a position directly below the measuring device B, the upper plunger 6 is moved downward and an upper plunger 6 is inserted into the cavity 4 through an upper opening of the cavity 4. Simultaneously with the above the lower plunger 5 also moved upward, the molding material 1 is compressed by the upper and lower plungers 5, 6 and preheated by a preheating device 3. The molding material 1 is compressed with gaps among particles becoming smaller and heat insulation effect by the gap also becomes smaller.