About 0.001 to 5 parts by weight of an antioxidant and about 0.001 to 10 parts by weight of a guanamine compound having at least one unit represented by the following formula (1) or a salt thereof relative to 100 parts by weight of a polyacetal resin are mixed and constitute a resin composition. The resin composition may further comprise a processing stabilizer, a heat stabilizer, a weather-resistant stabilizer, a coloring agent, a gloss control agent, an impact resistance improver, a slide improver, a filler, and/or the like. ##STR00001## Wherein R.sup.1 and R.sup.2 are the same or different and each represents a hydrogen atom or an alkyl group, and "m" denotes an integer of not less than 2.