A packaging material for a power storage device having a structure including at least a substrate layer, an adhesive layer, a metal foil layer, a sealant adhesive layer, and a sealant layer laminated in this order. In the packaging material, the substrate layer is formed of a polyester film having a 50% elongation stress in the range of 100 to 180 MPa and a thermal shrinkage in the range of 1 to 15%, after heat treatment from 160° C. to 200° C., or a polyester film having a difference ΔA in break elongation of 12% or more after heat treatment at 200° C. and 160° C. and having a 50% elongation stress of 75 MPa or more after heat treatment at 200° C.