PURPOSE: To obtain the titled composition having excellent long-term stability, especially heat-aging resistance and hot-water resistance, by compound an alkaline earth metal salt of a tribasic inorganic acid of a glycerol monofatty acid ester as a stabilizer.

CONSTITUTION: 100pts.wt. of an oxymethylene copolymer composed of 100mol of the oxymethylene unit of formula I (n is ≥1) and 0.05W50mol of randomly bonded oxyalkylene unit of formula II (R1 and R2 are H, etc.; n is 2W6) is compounded with 0.01W5pts.wt., preferably 0.03W1pt.wt. of an alkaline earth metal (e.g. magnesium) salt of a tribasic inorganic acid (e.g. phosphoric acid, boric acid, etc.) of a glycerol monofatty acid ester (e.g. ester derived from glycerol and a 8W22C fatty acid). The metallic salt is expressed by the formula III (R3 is 7W21C alkyl; X is ≡B, etc.; Y is alkaline earth metal), etc.