PURPOSE: To provide a high-quality vinyl chloride-based polymer esp. few in fish-eye and excellent in thermal stability.

CONSTITUTION: The polymer can be obtained by suspension polymerization in a polymerizer of an aqueous suspension containing (A) vinyl chloride or its mixture with another monomer and (B) an aqueous emulsion of an oil-soluble initiator. Specifically, said aqueous emulsion is first prepared, and a suspending agent being at least part of said agent used for preparing the above aqueous suspension and at ≥50wt.% per the oil-soluble initiator in said aqueous emulsion is then mixed with said aqueous emulsion; the resultant mixed liquor is fed into the polymerizer where the aqueous suspension is prepared and the suspension polymerization is initiated.