To give redundancy necessary for a power supply system in relation to a predetermined load on a low-voltage power source side by supplying the necessary voltage to the predetermined load on the low-voltage power source side without passing through a DC-DC converter.

In a power source device for a vehicle, a low-voltage power source 12 and a high-voltage power source 14 are connected to input and output sides of the DC/DC converter 100, respectively. The high-voltage power source has a low-voltage terminal 14b between a high-voltage terminal 14a and the ground, and a power supply line 70 from the low-voltage terminal is directly connected to the predetermined load 31 on the low-voltage power source side without passing through the DC-DC converter.