A cell main body and a control portion are tightly stored within a case main body, which is provided with a joint connector for electrical connection to the cell main body and the control portion. A nut member is disposed to surround this joint connector, which is attached for feeding to a feed connection portion for external power source connection of an electric reel. The nut member is screwed around a ferrule portion of the feed connection portion, and the case main body is mounted on the electric reel in a state where an engaged portion of the nut member is engaged by an engagement portion of the joint connector.