Process for producing vinyl chloride-based polymer

A process for producing a vinyl chloride-based polymer, comprising a suspension polymerization of either vinyl chloride, or a monomer mixture of vinyl chloride and another copolymerizable monomer, in an aqueous medium, and in presence of a dispersion stabilizer, wherein

• said dispersion stabilizer comprises (A) hydroxypropylmethylcellulose with a degree of methoxy group substitution of 26 to 30% by mass, a degree of hydroxypropoxy group substitution of 4 to 15% by mass, and a viscosity at 20° C. for a 2% by mass aqueous solution thereof of 5 to 4000 mPa·s, (B) a partially saponified polyvinyl alcohol with a saponification degree of 75 to 85 mol % and an average polymerization degree of 1000 to 3000, and (C) a partially saponified polyvinyl alcohol with a saponification degree of 20 to 57 mol % and an average polymerization degree of 150 to 600,
• said suspension polymerization is initiated in presence of said (A), (B), and (C), and respective quantities of said (A), (B), and (C) at initiation satisfy conditions prescribed by a formula (1) and a formula (2) shown below:
• mass of (A)/mass of (B)=2 or greater (1)
• and when the polymerization conversion is within a range from 15 to 50%, an additional quantity of said (C) is added to said polymerization reaction system.