PURPOSE: To prevent color smeet and to stably form a high quality color image by specifying the difference between the max. and min. values of the coefft. of thermal expansion in the surface of a film and the max. value.

CONSTITUTION: This polyester film for an image forming transfer material has Δat ≤1.0×10-5/°C difference betwen the max. and min. values of the coefft. of thermal expansion at in the surface and ≤2.5×10-5/°C of at max. value. The polyester is obtd. using arom. dicarboxylic acid or its ester and glycol as principal starting materials and, e.g. has ethylene terephthalate units or ethylene-2,6-naphthalate units by ≥80% of the repeating structural units. Isophthalic acid, may be used as the arom. dicarboxylic acid in place of terephthalic acid or 2,6-naphthalenedicarboxylic acid.