A battery device that prevents mistaken connection to a main apparatus includes a casing which houses a battery. The casing has a loading surface section on a lateral surface of its outer periphery. The loading surface section corresponds to a setting surface section of a main apparatus. The loading surface section of the casing has a discrimination groove having an opening end opened on an abutment surface and extending from the opening end parallel to the loading direction on the main apparatus. The abutment surface is formed on the loading surface of the casing for extending in a direction at right angles to the loading surface section.