Sealing film and outer packaging material for electricity storage device using same

Provided are: a sealing film which has high heat seal strength and excellent drawing formability, can maintain the amount of a lubricant on the surface of the film within an appropriate range even when the sealing film is cured after being laminated, and can achieve slippery properties and high lamination strength; and an outer packaging material for an electricity storage device, which uses the sealing film. A sealing film which is a three-layer laminated film of a surface layer A, a substrate layer B, and a surface layer C, and in which a sea-island structure is observed in at least one of the surface layer A and the surface layer C as observed from an image acquired using a transmission electron microscope in a cross-section cut parallel to the film longitudinal direction, and in the sea-island structure, the thickness of the surface layer A and the surface layer C is less than 1 [mu] m2 in cross-sectional image per 16 [mu] m2, and the thickness of the surface layer C is less than 1 [mu] m2 in the cross-se