PURPOSE: To obtain a polymer having highly isotactic stereoregularity by adding a definite amount of water to Mg ion in a Grignard reagent when methyl methacrylate is subjected to anionic polymerization.

CONSTITUTION: In producing a highly isotactic polymethyl methacrylate using methyl methacrylate by anionic polymerization, water is added to a polymerization system in an amount of ≥0.3 times by mol based on Mg ion in the Grignard reagent. Water which is allowed to coexist in the polymerization system can be added by directly adding to methyl methacrylate solution or bubbling hydrous nitrogen. The polymerization reaction is preferably carried out at low temperature and terminated by methanol.