PURPOSE: To prevent the available factor of a polymerizer from decreasing by detecting abnormal polymerization in its initial stage to thereby prevent a bulk polymer from being yielded and thus eliminate the necessity for bulk polymer removal.

CONSTITUTION: Vinyl chloride monomer alone or a mixture thereof with a copolymerizable monomer is suspension-polymerized, during which the change with time of the number of polymer particles in the suspension and/or the polymer particle size distribution is monitored to detect abnormal polymerization in its early stage. A suspension stabilizer or a polymerization inhibitor is added to the suspension as soon as any abnormal polymerization is detected to prevent a bulk polymer from being yielded.