To obtain an excellent ethylene-vinyl chloride copolymer having a little generation (adhesion) of scale on the inner wall of a reaction vessel during polymerization and useful as a raw material for a saponification product of an ethylene-vinyl acetate copolymer by dissolving a polymerization catalyst in vinyl acetate monomer in advance and then copolymerizing the monomer with ethylene.

(A) A polymerization catalyst [e.g. 2,2'-azobis-(2,4- dimethylvaleronitrile)] is dissolved, in advance, in (B) vinyl acetate monomer, subsequently the mixture is fed to a reaction system, and it is allowed to copolymerize with ethylene. Further, it is preferred that the amount of the component A is 0.002-50 pts.wt. based on 100 pts.wt. of the component B, the feeding temperature of the component B, in which the component A has been dissolved, to the reaction system is lower than the 1000-hour half life temperature of the component A; for example, a reaction is carried out commonly at a polymerization temperature of 40-80°C for about 4-10 hr to obtain the rate of polymerization (vinyl acetate) in the range of 20-90%.