The present invention provides a method of manufacturing a stabilized polyacetal resin, comprising the step of applying a heat treatment to a polyacetal resin having an unstable terminal group in the presence of an unstable terminal group decomposer selected from the following Group I, Group II and Group III, thereby decreasing the unstable terminal group: unstable terminal group decomposer of Group I: (I) a quaternary ammonium salt of an acidic cyclic amide compound, unstable terminal group decomposer of Group II: (II) a quaternary ammonium salt of a polycarboxylic acid intramolecularly having four or more carboxyl groups and/or a quaternary ammonium salt of an aminocarboxylic acid intramolecularly having one or more carboxyl group(s), unstable terminal group decomposer of Group III: (III) a quaternary ammonium salt, wherein a compound providing a counter-anion of a quaternary ammonium salt is at least one selected from the group consisting of the following (i) to (v): (i) a carbonic acid monoester and/or a metal bicarbonate, (ii) an acidic enol-based compound, (iii) a phenol-based and/or an alcohol-based compound, (iv) an acidic azole-based compound, and (v) at least one non-carboxylic acid-based compound selected from the group consisting of an organic sulfur compound, an organic phosphorus compound and an organic boron compound.