An electric storage cell has an electric storage element and a covering film package. The covering film package houses the electric storage element and includes: a metal layer having a first principle face on the electric storage element side and a second principle face on the opposite side of the first principle face, an internal resin layer made of synthetic resin and laminated to the first principle face, and an external resin layer made of synthetic resin and laminated to the second principle face, with a slit formed at least in the external resin layer; wherein a stress mitigation part to mitigate any stress that tries to separate the internal resin layers that are contacting each other, and a stress concentration part where this stress concentrates, are provided between the slit and the electric storage element.