PURPOSE: To prepare vinyl chloride (VC) polymer free from scale deposits, by polymerizing VC in an aqueous medium, using a polymerizer comprising agitating blades located in a quide cylinder spaced a little clearance.

CONSTITUTION: The agitating blades 6 are equipped at the center of the polymerizer 2, and the guide cylinder 4 (draft tube) is attached to the support 5 of the cylinder 4 at a little clearance from the blades 6. Nothing is limited about the ratio of the radius of the cylinder 4 to that of polymerizer 2; however, it is preferably about 0.5W0.9. VC or a mixture consisting mainly of VC is polymerized in an aqueous medium by means of the polymerizer to deposit scales scarcely on the inner wall of the polymerizer, the guide cylinder 4, and the blades 6.