A polyester film for forming which is excellent in formability especially at low temperatures and low pressures and in solvent resistance and heat resistance and is reduced in environmental burden. The polyester film for forming is a biaxially oriented polyester film comprising a copolyester as a component, and is characterize in that (1) the stress at 100% elongation of the film in each of the machine direction and the transverse direction is 10 to 1,000 MPa at 25° C. and is 1 to 100 MPa at 100° C.; (2) the storage viscoelasticity (E′) of the film in each of the machine direction and the transverse direction is 10 to 1,000 MPa at 100° C. and is 5 to 40 MPa at 180° C.; and (3) the degree of heat distortion (initial load, 49 mN) of the film in the machine direction is −3% to +3% at 175° C.