Abstract: Disclosed is a method for stabilizing a crude oxy-methylene copolymer having thermally unstable terminal groups, which comprises subjecting a crude oxymethylene copolymer to heat treatment in the presence of at least one quaternary ammonium compound represented by the following formula (1): [R1R2R3R4N+]nX−n  (1) wherein each of R1, R2, R3 and R4 independently represents an unsubstituted or substituted alkyl group, an aryl group, an aralkyl group which is an unsubstituted or substituted alkyl group substituted with at least one aryl group, or an alkylaryl group which is an aryl group substituted with at least one unsubstituted or substituted alkyl group; n is an integer of from 1 to 3; and X is a hydroxyl group or an acid residue. In this method, even by the addition of a very small amount of the quaternary ammonium compound, the decomposition rate of the unstable terminal groups can be remarkably increased, thereby facilitating the stabilization of the crude copolymer.