In a method for mixing a trioxane, a polymerization catalyst and a comonomer for producing of oxymethylene copolymers by continuously copolymerizing trioxane with a cyclic ether or a cyclic acetal as a comonomer, the improved method comprises following steps:spouting a comonomer and a polymerization catalyst from nozzle openings which are set adjacent to each other,while washing both openings at the tip end of the nozzles with a flow of trioxane,whereby mixing said comonomer, said polymerization catalyst and said trioxane,and then feeding the mixture to a polymerization reactor to effect copolymerization.The raw materials are mixed together before they are fed to the polymerization reactor, and no clogging trouble at the tip end of the nozzle for feeding a polymerization catalyst occurs. Further the resulting copolymer has a much superior heat stability. In addition, since each of the reaction raw materials is quantititively fed, the molecular weight and conversion of the copolymer extended from the polymerization reactor is so constant that a stabilized operation of the polymerization reactor can be secured.