Battery side terminals disposed on the first and second battery devices are arranged at the same places at front surfaces with respect to lower surfaces and side surfaces. On the front surface of a case of the second battery device, there are formed engaging recessed parts. Positions close to the lower surface of the engaging recessed parts are formed as flat surfaces parallel to a upper surface and the lower surface. The dimensions from the lower surface of the case to flat surfaces and the dimension from the lower surface of the case of the first battery device to an upper surface are configured to be the same. Accordingly, it is possible to provide an advantageous battery device that allows the use of battery devices having various capacities and improves the ease of use.