A power storage device packaging material, 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 metal foil layer has a thickness in the range of 9 to 100 μm, characterized in that the substrate layer has the following characteristics (i) and (ii): (i) the substrate layer is formed of a polyester film exhibiting ΔA, as expressed by the following Formula 1, of 10% or more and a 50% elongation stress of 75 MPa or more after heat treatment at 160°C: Δ A = break elongation after 160 ° C heat treatment-break elongation before 160 ° C heat treatment wherein the break elongation and the 50% elongation stress are each an average of tensile tests at 23°C in four directions of the substrate layer, the four directions are the mechanical feed direction MD, the direction in 45° to MD, the direction in 90° to MD which is the transverse direction TD, and the direction in 135° to MD, and the following parameters are applied in the tensile test: test piece shape: dumbbell No. 5 according to JIS K7127, length between chucks: 75 mm, gauge length: 25 mm, test speed: 50 mm/min; (ii) the substrate layer is formed of a polyester film having a piercing strength of 0.6 N/μm or more. 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 metal foil layer has a thickness in the range of 9 to 100 μm, characterized in that the substrate layer has the following characteristics (i) and (ii): (i) the substrate layer is formed of a polyester film exhibiting ΔA, as expressed by the following Formula 1, of 10% or more and a 50% elongation stress of 75 MPa or more after heat treatment at 160°C: Δ A = break elongation after 160 ° C heat treatment-break elongation before 160 ° C heat treatment wherein the break elongation and the 50% elongation stress are each an average of tensile tests at 23°C in four directions of the substrate layer, the four directions are the mechanical feed direction MD, the direction in 45° to MD, the direction in 90° to MD which is the transverse direction TD, and the direction in 135° to MD, and the following parameters are applied in the tensile test: test piece shape: dumbbell No. 5 according to JIS K7127, length between chucks: 75 mm, gauge length: 25 mm, test speed: 50 mm/min; (ii) the substrate layer is formed of a polyester film having a piercing strength of 0.6 N/μm or more. (i) the substrate layer is formed of a polyester film exhibiting ΔA, as expressed by the following Formula 1, of 10% or more and a 50% elongation stress of 75 MPa or more after heat treatment at 160°C: Δ A = break elongation after 160 ° C heat treatment-break elongation before 160 ° C heat treatment wherein the break elongation and the 50% elongation stress are each an average of tensile tests at 23°C in four directions of the substrate layer, the four directions are the mechanical feed direction MD, the direction in 45° to MD, the direction in 90° to MD which is the transverse direction TD, and the direction in 135° to MD, and the following parameters are applied in the tensile test: test piece shape: dumbbell No. 5 according to JIS K7127, length between chucks: 75 mm, gauge length: 25 mm, test speed: 50 mm/min; wherein the break elongation and the 50% elongation stress are each an average of tensile tests at 23°C in four directions of the substrate layer, the four directions are the mechanical feed direction MD, the direction in 45° to MD, the direction in 90° to MD which is the transverse direction TD, and the direction in 135° to MD, and the following parameters are applied in the tensile test: test piece shape: dumbbell No. 5 according to JIS K7127, length between chucks: 75 mm, gauge length: 25 mm, test speed: 50 mm/min; (ii) the substrate layer is formed of a polyester film having a piercing strength of 0.6 N/μm or more.