A process for polymerizing trioxane made from an aqueous solution of methanol-containing formaldehyde by a conventional method or copolymerizing the trioxane with another monomer, characterized by cooling the trioxane to crystallize it, allowing the trioxane in a crystalline state to stand in an open system in an atmosphere of air or an inert gas to remove polyoxymethylene dimethoxide from the trioxane, melting the trioxane, and polymerizing the trioxane or copolymerizing the trioxane and the another monomer.
