To provide a polyester film suitable for a substrate to form a film interlayer insulation material, which is used for a multilayer printed wiring board manufactured using a build-up method to correspond to fine division and densification of the wiring.

This polyester film substrate for interlayer insulation is used as the substrate on which a thermosetting resin is applied to form an interlayer insulation layer, wherein a degree of thermal shrinkage of the film in a width direction after 120°C×5 min is in a range of 0-1.0%, and thickness of the film is in a range of 20-100 μm.