A method for producing an isocyanate group-blocked liquid urethane prepolymer which comprises: reacting a lactone-modified diol (3) with a polyfunctional isocyanate (4) to form an isocyanate group-having urethane prepolymer (5), wherein said lactone-modified diol (3) is obtainable by ring-opening addition polymerization of a lactone monomer (2) with a diol (1) selected from the group consisting of a polyalkylene ether diol, a polyester diol and a polycarbonate diol; and then reacting said urethane prepolymer with a blocking agent (6).