A cap for closing an opening of a battery case for containing an electrode assembly is formed by an elongated plate extending in a longitudinal direction. The cap includes: a terminal hole for allowing a negative electrode terminal of the electrode assembly to pass therethrough; a recess formed in an outer surface of the cap, the outer surface representing the exterior of the battery, the recess located in a region that surrounds the terminal hole and that is not in contact with the outer periphery of the outer surface. An edge of the recess includes a projection protruding inwardly. The projection is positioned to overlap the terminal hole as determined along the longitudinal direction.