A high-quality polyacetal copolymer produced by a simple process in an economical manner. The process includes supplying a raw material including trioxane and the like to a reaction device; setting the polymerization environmental temperature to no more than 100° C. until the reaction device conversion rate becomes 0.5, and then carrying out further polymerization; vaporizing and separating unreacted monomers from the reaction mixture at an environmental temperature of at least 115° C. and less than 140° C.; supplying the separated monomers to the raw material supply; and recovering the polyacetal copolymer from the reaction mixture.