To provide a method for producing a new N-alkylmaleimide-based polymer which has excellent heat resistance and transparency and is suitable for an optical material.

The method for producing the N-alkylmaleimide-based polymer is characterized by suspension-polymerizing a monomer mixture comprising 50-100 wt.% N-alkylmaleimide expressed by general formula (1) and 0-50 wt.% other copolymerizable monomer, in an aqueous medium in the presence of water soluble cellulose by using an oil-soluble radical polymerization initiator.