PURPOSE: To accurately perform decision of closing failure of contact within a short period as a means for preventing short-circuit of a battery. in a power supply voltage switching apparatus for vehicle.

CONSTITUTION: A closing failure of a contact 4b is detected by measuring a terminal voltage Va of a feeding switch 6 under the condition that each contact is all opened after the power supply to a large power load 5 in the serial connecting condition. In this case, charges accumulated at the one terminal of the feeding switch 6 and capacitance element of the wirings are discharged through the large power load 5 by closing the feeding switch 6 for a short period. Thereby, mis-judgement for normal contact as a closing failure due to the charges accumulated in the one terminal of the feeding switch 6 and capacitance element of the wirings can be prevented.