Polyester film and method for producing same

A polyester film having a layer (L1) and a layer (L2),

• wherein the layer (L1) comprises a polyester (A) composed mainly of polybutylene terephthalate, a polyester (B) composed mainly of polyethylene terephthalate, and a low-molecular weight polymer having a weight average molecular weight (Mw) of 30,000 or less, and
• wherein the layer (L2) comprises the polyester (A) and the polyester (B), does not comprise the low-molecular weight polymer,
• wherein a mass ratio (A/B) between the polyesters (A) and (B) in the polyester film is 70/30 to 55/45,
• wherein the first dry heat shrinkage rate of the film through heat treatment of the film at 200° C. for 15 minutes is 35% or less in any of four directions on a film surface consisting of a 0° direction as an arbitrary direction, and a 45° direction, a 90° direction and a 135° direction clockwise from the 0° direction, and a difference between a maximum value and a minimum value of these dry heat shrinkage rates is 5% or less,
• wherein a thickness variation calculated from the following equation in the four directions is 10% or less:
• Thickness variation (%)=(Tmax−Tmin)/Tave×100
• Tmax:
• maximum thickness in four directions of polyester film
• Tmin:
• minimum thickness in four directions of polyester film
• Tave:
• average thickness in four directions of polyester film, and
• wherein a crystallization index shown in DSC measurement is 25 to 55 J/g.