A packaging material for a power storage device comprising: 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, wherein: the substrate layer is formed of a polyester film, wherein the polyester film has the following features (i) or the following features (ii): (i) a 50% elongation stress in a range of 100 to 180 MPa and a thermal shrinkage in a range of 1% to 15%, after heat treatment in a range of 160°C to 200°C, wherein the thermal shrinkage is an average of shrinkages before and after heat treatment in four directions (0° (MD), 45°, 90° (TD), 135°) of the substrate layer; (ii) an ΔA of 12% or more expressed by Formula below and a 50% elongation stress of 75 MPa or more after heat treatment at 200°C: Δ A = break elongation after 200 ° C heat treatment − break elongation after 160 ° C heat treatment wherein the break elongation and the 50% elongation stress are each an average at 23°C resulting from tensile tests (test piece shape: dumbbell No. 5 according to JIS K7127; length between chucks: 75 mm; gauge length: 25 mm; test speed: 50 mm/min) in four directions (0° (MD), 45°, 90° (TD), 135°) of the substrate layer, characterized in that the power storage device further comprises an adhesion enhancement layer provided between the substrate layer and the adhesive layer. 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, wherein: the substrate layer is formed of a polyester film, wherein the polyester film has the following features (i) or the following features (ii): (i) a 50% elongation stress in a range of 100 to 180 MPa and a thermal shrinkage in a range of 1% to 15%, after heat treatment in a range of 160°C to 200°C, wherein the thermal shrinkage is an average of shrinkages before and after heat treatment in four directions (0° (MD), 45°, 90° (TD), 135°) of the substrate layer; (ii) an ΔA of 12% or more expressed by Formula below and a 50% elongation stress of 75 MPa or more after heat treatment at 200°C: Δ A = break elongation after 200 ° C heat treatment − break elongation after 160 ° C heat treatment wherein the break elongation and the 50% elongation stress are each an average at 23°C resulting from tensile tests (test piece shape: dumbbell No. 5 according to JIS K7127; length between chucks: 75 mm; gauge length: 25 mm; test speed: 50 mm/min) in four directions (0° (MD), 45°, 90° (TD), 135°) of the substrate layer, characterized in that the power storage device further comprises an adhesion enhancement layer provided between the substrate layer and the adhesive layer. (i) a 50% elongation stress in a range of 100 to 180 MPa and a thermal shrinkage in a range of 1% to 15%, after heat treatment in a range of 160°C to 200°C, wherein the thermal shrinkage is an average of shrinkages before and after heat treatment in four directions (0° (MD), 45°, 90° (TD), 135°) of the substrate layer; (ii) an ΔA of 12% or more expressed by Formula below and a 50% elongation stress of 75 MPa or more after heat treatment at 200°C: Δ A = break elongation after 200 ° C heat treatment − break elongation after 160 ° C heat treatment wherein the break elongation and the 50% elongation stress are each an average at 23°C resulting from tensile tests (test piece shape: dumbbell No. 5 according to JIS K7127; length between chucks: 75 mm; gauge length: 25 mm; test speed: 50 mm/min) in four directions (0° (MD), 45°, 90° (TD), 135°) of the substrate layer, characterized in that the power storage device further comprises an adhesion enhancement layer provided between the substrate layer and the adhesive layer. <EOS>