To provide a method of manufacturing a polyacetal resin composition which excels in a balance of mechanical properties, such as heat stability, rigidity, and toughness, and excels in creep resistance life, fatigue resistance, and further acid resistance.

The method of manufacturing a polyacetal resin composition includes a process which melt kneads: based on (I) 100 pts.mass of a polyacetal resin; (II) over 5 pts.mass and under 100 pts.mass of light calcium carbonate in which an average particle diameter is from 0.1 μm to less than 1.5 μm, and an average aspect ratio (L/D) which is a ratio of an average major axis (L) of a particle to an average minor axis (D) of a particle is at most 3, and a surface treatment is not performed by a 8-32C saturated fatty acid, unsaturated fatty acid, and a metal salt thereof; and (III) 0.005-10 pts.mass of an organic acid, simultaneously at above a melting point of the polyacetal resin (I).