An electric power unit selectable and usable a plurality of electric sources, including an electric source-discriminating device to detect attached electric sources of the plurality of electric sources, an electric source-selecting device to select one of the attached electric sources as a using electric source, at least one discharging device to discharge residual electric charges on electric source lines from the electric sources to the electric source-selecting device and/or electric source lines over the electric source-selecting device, and a control device configured to control the discharging device to discharge residual electric charges on the electric source lines where the electric sources are not connected or all electric source lines for a predetermined time or to a predetermined voltage, the electric source-discriminating device to discriminate the attached electric sources, and the electric source-selecting device to select the discriminated electric sources as the using electric source.