PURPOSE: To prepare a homogeneous vinyl polymer, by circulating a portion of a polymer-containing luquid, discharged from a polymerization zone, into a polymerization material system, for polymerization.

CONSTITUTION: A stock liquid for polymerization, composed of a vinyl monomer containing at least 80wt% of methyl methacrylate, a polymerization initiator and a polymerization regulator, is supplied through heat exchanger 6, by pump 3, from tank 1 to mixer 8, where it is mixed with a polymer-containing liquid, discharged from reactor 10, and then the mixture is introduced into reactor 10, where it is polymerized at a regulated temperature of 100W200°C, to an extent of polymer content 50W80wt%. The polymer-containing liquid is sent through line 11, pump 12, viscometer 22 and line 13. A portion of the liquid is circulated, through line 14 and heat exchanger 23, to mixer 8. The residual portion is supplied to volatiles separator 15, where the monomer is separated, and the polymer is discharged through 17. The monomer is totally recovered at total condenser 19, after separation from impurities in separator 15, and recycled to tank 1 through holder 20.