The present invention provides a substrate for a flexible printed wiring board including an adhesive layer containing an epoxy resin composition, insulating layers respectively stacked on both sides of the adhesive layer and formed with a pair of films containing a nonthermoplastic polyimide resin, and conductor layers respectively disposed on the outer surfaces of the films. The total thickness of the insulating layers respectively stacked on both sides of the adhesive layer is 10 to 100 .mu.m and 2 to 10 times the thickness of the adhesive layer. The mutual adhesion strength between the insulating layers through the intermediary of the adhesive layer is 7.0 N/cm or more.