A main switch with a low withstand voltage usable to prevent a leak current from flowing from a floated running power supply by way of the main switch even when the insulation between the main switch and the body grounding is reduced. A running power supply for supplying power to a running motor, which includes a plurality of batteries connected in series to each other, is kept in the floating state with respect to the body grounding. Moreover, there is provided a step-down DC-DC converter of an input-output insulation type. A diode and a main switch are provided from a battery constituting part of the running power supply in this order to supply a starting power or starting signal to the DC-DC converter for starting the DC-DC converter. Thus, power is supplied to the input side of the DC-DC converter from the running power supply, to obtain a step-down accessary power supply VB.