A method of producing a medical polyoxypropylene polymer and a polyoxypropylene/polyoxyethylene block copolymer including (A) adding to a polyoxypropylene polymer which is obtained by ring-opening polymerization of propylene oxide to a starting substance having an active hydrogen reacting with the propylene oxide and contains allyl ether as an impurity, a tertiary alkoxide of alkali metal in an excess amount based on a molar number of the active hydrogen of the starting substance and heat treating at 115° C. or less to isomerize the allyl ether to propenyl ether； and (B) adding a mineral acid to the product obtained in step (A) to adjust pH to 4 or less and treating at 70° C. or less to hydrolyze the propenyl ether. Also disclosed is a method of producing a medical polyoxypropylene/polyoxyethylene block copolymer which includes performing ring-opening polymerization of ethylene oxide to the polyoxypropylene polymer obtained above.
