A method for producing a high molecular weight polyester having swell of 1-200 percent is described. In the method, a mixture consisting of (a) 100 parts by weight of a linear saturated polyester; (b) 0.3˜10 parts by weight of a mixture, as a coupling agent, which is composed of (1) 0-100 percent by weight of a bifunctional epoxide ingredient containing 2 epoxide groups per molecule and (2) 100-0 percent by weight of a polyfunctional epoxide ingredient containing 3 and more epoxide groups per molecule; and (c) 0.01˜5 parts by weight of metal salt of carboxylic acid as a catalyst for coupling reaction, is heated at a temperature higher than a melting point of said polyester resin.