This power storage device includes: a power storage element; a case having an opening and accommodating the power storage element; and a sealing member for sealing the opening. The sealing member includes an elastic member fitted in the opening. The elastic member includes an elastic polymer and a hindered phenol compound. The hindered phenol compound has a first hindered group and a second hindered group respectively at the first substitution position and the second substitution position adjacent to a phenolic hydroxy group in a phenol skeleton. The first hindered group and the second hindered group each include at least one tertiary carbon atom, and one tertiary carbon atom of the first hindered group is bonded to the first substitution position and one tertiary carbon atom of the second hindered group is bonded to the second substitution position.