Disclosed is an oxytetramethylene glycol copolymer, which is obtained by copolymerizing tetrahydrofuran and neopentyl glycol and which has a specific number average molecular weight Mn, a specific molecular weight distribution Mw/Mn, a specific neopentyl glycol copolymerization whole ratio N.sub.w and a specific neopentyl glycol copolymerization partial ratio N.sub.h, and a method for producing the same. Also disclosed is a method for purifying an oxytetramethylene glycol copolymer, which comprises adding a fresh tetrahydrofuran to a reaction mixture comprising an oxytetramethylene glycol copolymer and an unreacted diol, wherein the amount of added tetrahydrofuran is not less than the weight of the unreacted diol contained in the reaction mixture, and distilling off the unreacted diol together with the added tetrahydrofuran.