To improve thermal stability and molding stability of a polyacetal resin thereby inhibiting the evolution of formaldehyde.

Into 100 pts.wt. of powdery and granular materials of a polyacetal resin are incorporated around 0.001-10 pts.wt. of an inhibitor composed of an active hydrogen-containing compound. The active hydrogen- containing compound is at least one selected from the group consisting of ureas (N-substituted ureas, urea condensates and the like) and amidines [N-substituted derivatives of amidines, guanidines (cyclic guanidines and non-cyclic guanidines) and the like]. As the polyacetal resin can be employed a weathering resistant or light resistant polyacetal resin and a colorant may be added to the polyacetal resin.