PURPOSE: To improve respectively transparency, water absorption properties and mechanical properties, by a method wherein dicarboxylic acid copolymer component of polyester constituting at least one layer of a polyester film is constituted of aromatic dicarboxylic acid and polyether dicarboxylic acid.

CONSTITUTION: In a polyester film possessing multi-layer structure, a dicarboxylic acid copolymer component of polyester (A) constituting at least its one layer is constituted as follows. In other words, aromatic dicarboxylic acid and/or its derivative (B) having metal sulfonate and polyether dicarboxylic acid and/or its derivative (C) having mean molecular weight of 600-20000 as shown by a formula as mentioned elsewhere are caused to be contained respectively. In the formula as mentioned elsewhere at this time R1 and R3, R2 and n indicate respectively an H or 1-8C alkyl group, 2-8C alkylene group and positive integral number. Then, in the above-mentioned polyester, (B) and (C) are established at the rate of 2-20wt.% and 2-20wt.% respectively.