An aldehyde inhibitor composition for inhibiting the generation of aldehyde from an aldehyde formation source, which aldehyde inhibitor composition comprises a carboxylic acid hydrazide and, mixed with 1 part by weight thereof, 0.01 to 100 parts by weight of metal hydroxypolycarboxylate (for example, salts of citric acid, malic acid or tartaric acid with alkaline earth metals). Further, there maybe provided a polyacetal resin composition comprising a polyacetal resin and, mixed with 100 parts by weight thereof, 0.001 to 20 parts by weight of the above inhibitor composition. This resin composition may be loaded with an antioxidant, a thermal stabilizer, a processing stabilizer, a weather (light) stabilizer, an impact resistance enhancer, a gloss control agent, a sliding property improver,a colorant, a filler, etc. The above aldehyde inhibitor composition when used in combination with a polyacetal resin can also effectively inhibit the generation of aldehyde with the addition of a small amount thereof without causing any resin discoloration.