To simplify the construction of both first and second d.c. power supply units connected to a post-stage part by interposing a constant voltage circuit outputting a constant voltage not more than an output voltage of the first d.c. power supply unit between a battery mounted on a vehicle and the first and second d.c. power supply units.

An output of a constant voltage control circuit 12 is set within an allowable voltage range for an on-vehicle battery 1. At the constant voltage control circuit 12, an output voltage is set to correspond with an output voltage of a DC/DC converter 2, and thereby there occurs no loss of power caused by the DC/DC converter 2. A voltage decreasing circuit 13 is set so that it outputs a constant voltage of e.g. 6V slightly higher than an output voltage of a 5V regulator 9. When a voltage split by resistors 13f, 13g is smaller than a reference voltage of a comparator 13e, the output is made low in level and a control transistor 13a is put on, then a smoothing capacitor 13d is charged via a coil 13c.