To economically produce a polyacetal copolymer excellent in properties including thermal stability by copolymerizing trioxane as the main monomer through simple steps, a polyacetal copolymer is produced by copolymerizing trioxane as the main monomer using a nonvolatile protonic acid catalyst as a polymerization catalyst until the conversion reaches at least 60% (based on all monomers), and then vaporizing unreacted monomers to separate, remove, and recover the same from the polymerization system to thereby reduce the residual monomer content of the polymerization system to 5% by weight or lower.