The present invention provides a battery including a case and a battery cell accommodated in the case. The case is divided in a thicknesswise direction of the battery cell into a first case and a second case. The first and second cases have joint portions formed along peripheries thereof for jointing with each other. The joint portion of one of the first and second cases has a joining thin wall formed to extend along a periphery thereof and project in the thicknesswise direction. The joint portion of the other of the first and second cases has a joining recess formed to extend along a periphery thereof for receiving the joining thin wall inserted therein in a state that the joint portions of the first and second cases joint with each other. The joining thin wall and the joining recess in which the joining thin wall is inserted while the joint portions of the first and second cases joint with each other are joined together by welding. The other case has a projecting wall formed at a location of an outer face thereof positioned on the outer side of the joining recess in such a manner as to extend along the joining recess.