PURPOSE: To obtain a polyester film superior in low-temperature shrinkable properties without impairing excellent characteristics of polyester, by a method wherein a kind and quantity of a copolymeric monomer of a polyethylene terephthalate copolymer and contents of diethylene glycol are specified.

CONSTITUTION: A polymer for this shrinkable film consists mainly of terephthalic acid and ethylene glycol, isophthalic acid (A) and a compound (B) which is shown by a formula I (R represents formulas II, III) are copolymerized at mol% satisfying relative formulas of 3≤A≤10, 1≤B≤10, 6≤A+B≤14 and contains 2-9mol% diethylene glycol (C). When a component A is at most 3mol% or a component A+B is at most 6mol%, a high-shrinkable film is not obtained and when the component A is at least 10mol% or the component A+B is at least 14mold%, a time change is generated on fusing at the time of drying of the tip or shrinking properties of a stretchding film. Then when the component C is at most 2mol%, crystallization is generated at the time of processing of the film and when the same is at least 9mol%, weather resistance is lowered.