To clarify the reaction mechanism of the solid-state polymerization of trioxane, the molecular weight distribution of polyoxymethylene is measured. High molecular weight polymer is precipitated around the stirrer in an early stage, then low polymer, while thep-chlorophenol solution of polyoxymethylene is violently rotated by a glass stirrer at about 60°C. To fractionate polymer, this phenomenon is applied for the polyoxymethylene produced in the solid-state polymerization catalyzed by BF3·O(C2H5)2inn-hexane medium at 40 and 50°C. The polymer obtained at a low conversion contains a large amount of high polymer part, and the amount of high polymer decreases with increasing conversion. It is estimated that high polymer is produced only at the initial stage of the polymerization and that the decrease of molecular weight of polymer at a high conversion in the solid-state polymerization may be due to the formation of low polymer at a high conversion and also to the degradation of high polymer during the polymerization.