PURPOSE: To obtain the titled compound capable of giving high-molecular weight polyoxymethylene in high purity with little formation of by-products without depositing paraformaldehyde nor corroding apparatuses, by bringing formaldehyde in the vapor phase into contact with a heteropolyacid supported on a carrier.

CONSTITUTION: A carrier, e.g. silica gel, is gently agitated in an aqueous solution of a heteropolyacid, e.g. phosphotungstic acid, at 80°C, and the carrier is then separated and dried at 120°C under reduced pressure to afford a catalyst containing the heteropolyacid supported on the carrier. The resultant catalyst is then packed in a stainless steel tube, etc. having a jacket, the stainless steel tube, etc. and brought into contact with the catalyst consisting of the heteropolyacid supported on the carrier and give the aimed trioxane of high purity.