To provide a suspension polymerization method for a vinyl chloride- based resin, free from adhesion of resin scale on the inner wall of a polymerizer, providing a vinyl chloride-based resin composition having a sharp particle size distribution and a bulk specific gravity, almost no skin layer part on the surface of the particles and extremely excellent moldability and processability.

This method for subjecting a vinyl chloride-based monomer to suspension polymerization in the presence of an oil-soluble polymerization initiator in an aqueous medium. A partially saponified polyvinyl acetate (a) having 60-80 mol.% saponification degree, 450-600 average polymerization degree and 3-10 cP viscosity (viscosity of 4 wt.% solution in a solvent of water/ methanol of 1/1 (weight ratio) and a measured value by a Brookfield viscometer at 20°C based on JIS K 6,726), at least one emulsifying agent (b) of a sorbitan higher fatty acid ester having 3-10 cP HLB value and an anionic emulsifying agent, a 8-25C higher fatty acid (c) and a water-soluble thicker (d) having 10-200 cP Brookfield viscosity at 0.1% aqueous solution at normal temperature under normal pressure are added to the reaction system.