PURPOSE: To prevent a resin from stagnating on a belt, shorten a screw, and eliminate a need for applying an excessive shearing force to a material in a screw type plasticizer provided with a material preheating device consisting of rolls and a belt.

CONSTITUTION: A shaft of a screw 2 of a screw type plasticizer is in parallel to shafts of rolls 9 of a material preheating device. The periphery of a belt 10 stretched between the rolls 9 is disposed in proximity to the peripheral circle of a screw flight 18. A contact P of the belt 10 with a scraper 17 mounted on a cylinder 1 is set so as to form a central angle α at the center of the roll 9 ranging within ±2.5° on each side of a straight line connecting the center of the roll 9 to the center of the screw 2.