A film-formable high molecular polyetherpolyester comprising structural unit (1) of formula (1) and functional group (2) of formula (2) and having a number-average molecular weight of 40,000 to 10,000,000, wherein the molar ratio of functional group (2) to R in the polyetherpolyester is in the range of 1.0 to 3.0. This polyetherpolyester can be produced by carrying out a chain extension reaction of a low molecular polyalkylene oxide of 1,000 to 30,000 in number-average molecular weight with a polyvalent acid anhydride at 0.003 MPa or higher in a molar ratio of (acid anhydride group)/(hydroxyl group)=0.8 to 1.0. ##STR1##