An electricity storage device includes a container which houses an electricity generating element, and a sealing plug which seals a liquid injecting hole into which an electrolyte is injected. The liquid injecting hole is provided in a bottom surface of a recessed portion provided to be recessed on one side surface of the container. The sealing plug includes an inserting portion to be inserted into the liquid injecting hole and a fitting portion to be fitted into the recessed portion. At least either a rim portion of the fitting portion or an opening rim portion of the recessed portion is provided with a plurality of plastic deformation portions. In each of n regions (n is an integer of at least 3) into which an outer circumference of the fitting portion or an inner circumference of the recessed portion is equally divided, one or more of the plastic deformation portion (s) is/are arranged, and, in each of regions into which the outer circumference of the fitting portion or the inner circumference of the recessed portion is halved with an arbitrary plastic deformation portion out of the plastic deformation portions set as a starting point, one or more of the plastic deformation portion(s) is/are arranged. An outer circumferential side surface of the fitting portion and an inner circumferential side surface of the recessed portion are welded over an entire circumference.