An outside gasket for insulating a cover from an external terminal to be fixed thereto is readily fixed to the cover irrespective of a simple structure. A battery includes: a battery case which contains a power generating element therein and whose top surface is opened; a cover which closes the top surface; at which an external terminal exposed to the outside is disposed at the outer surface of the cover via an outside gasket, the cover having an engagement portion which expands outward and is formed into a shape except a circle, and the outside gasket having a recess to be engaged to the engagement portion.