A battery apparatus including a case, a battery cell, a battery-side terminal, and an engaging piece. The case has a width, a thickness, and a length. The battery cell is housed in an inside of the case. The battery-side terminal is disposed on a side surface at one end of the case in a length direction and electrically connected to the battery cell. The engaging piece is disposed at the one end of the case at which the battery-side terminal is positioned such that the engaging piece extends in a length direction a same distance as the case at the one end portion. The engaging piece is at an edge of the case in a width direction.