A polyester film comprising a polyester whose dicarboxylic acid copolymerized component contains (i) an aromatic dicarboxylic acid having a metal sulfonate and/or a derivative thereof and (ii) a polyetherdicarboxylic acid represented by the following chemical formula (a) and having a mean molecular weight in the range of 600 to 20000 and/or a derivative thereof. R1OOCCH2-(O-R2)n -OCH2COOR3 (a) Where, "R1" and "R3" are H or alkyl groups each having a carbon number of 1 to 8, "R2" is alkylene group having a carbon number of 2 to 8 and "n" is a positive integer. The polyester film has excellent transparency, mechanical properties and dimensional stability such as those of a PET film and an excellent water absorption property such as that of a TAC film. Therefore, when the film is used as a base film for a photosensitive material, a good curl-extinguishing property as well as good transparency, mechanical properties and dimensional stability of the material can be realized.