To provide a method for producing a polyacetal resin excellent in the chemical permeation resistance.

This resin is produced by using a cationic polymerization catalyst and performing the bulk polymerization of (a) 100 pts.wt. trioxane, (b) 0.01-20 pts.wt. cyclic ether compound selected from ethylene oxide, 1,3-dioxolane, diethylene glycol formal and 1,4-butan diol formal and (c) 0.01-20 pts.wt. cationic copolymerizable compound except for (b) components. In the first step, (a) and (b), or (a), (b) and a part of (c) are subjected to the polymerization reaction to form a reactive intermediate and, in the second step, all or residue of (c) is added to the intermediate to perform the polymerization reaction.