PURPOSE: To eliminate leakage of high voltage to a vehicle body and to reduce the possibility of an electric shock due to the high voltage by separately providing ground potentials of high and low voltage DC circuits, connecting the low voltage directly to the body, and connecting the high voltage to a common potential terminal in an insulation box insulated from the body.

CONSTITUTION: A common potential terminal block 21 is provided in an insulation box 20 to wire high and low DC common potential via a covered wire. A cut-off switch 8 for cooperatively disconnecting the terminals of both the electrodes of a secondary battery circuit 3 is provided. Then, the block 21 is connected to the load side of the switch 8, and the box 20 is provided at a position which cannot be accessed if the battery is removed from a battery chamber for containing the circuit 3. Thus, the deterioration of the insulation of the common potential due to the use for a long period or influence of rainy water is prevented. This prevents the simple accessing of a user who is not trained, eliminates the possibility of contact of a high voltage DC circuit with a human body and further improves safety.