Method for producing polyacetal copolymer

A method for producing a polyacetal copolymer, comprising:

• supplying a raw material comprising trioxane, a comonomer which copolymerizes with the trioxane, and a non-volatile protonic acid, to a continuous stirring mixer type reaction device having two parallel shafts which rotate in the same or opposite directions, wherein a multitude of paddles is mounted on each shaft, and a barrel near an outer circumference of the paddles, wherein a long axis end of a paddle periodically comes close to a short axis end of a companion side, wherein the raw material is charged from an introduction opening provided at one end in an axial direction, and wherein a reaction mixture and an unreacted monomer are obtained from a plurality of removal openings provided at the other end;
• carrying out a polymerization reaction of the raw material in a state in which the rotating shafts are inclined by 1 to 6° upwards with respect to a horizontal direction from an input opening of the reaction device towards an output opening, and obtaining a reaction mixture;
• vaporizing and separating an unreacted monomer from the reaction mixture, and supplying the same to the raw material supply;
• and recovering the polyacetal copolymer from the reaction mixture,
• wherein a weight ratio D/C of the polyacetal copolymer D with respect to a total supplied monomer C which is defined as a sum of a monomer A which is newly supplied to the reaction device and a monomer B which is recovered from the reaction device and re-supplied, is no less than 0.7;
• and a weight ratio D/A of the polyacetal copolymer D with respect to the monomer A which is newly supplied to the reaction device is no less than 0.85.