PURPOSE: A mercaptan as a chain transfer agent is added by a prescribed amount before the start of polymerization and further continuously added by an specified amount to polymerize polyvinyl acetate of low polymerization degree, preferably of lower than 700, batchwise.

CONSTITUTION: A mercaptan as a chain transfer agent is added to the polymerization system (a vinyl acetate solution in methanol) before the start of the polymerization by such an amount as satisfying equation I ([S] is the concentration of mercaptan; [M]o is the initial concentration of vinyl acetate; the unit of the concentration is mol/1; Cx is the chain transfer constant of mercaptan; P is the target of polymerization degree). At the same time when the polymerization is started, further mercaptan is added to the polymerization system continuously at such a rate as shown by equation II (Rpo is initial polymerization rate in mol/1/sec) to effect the polymerization of vinyl acetate.