A vehicle power-supply unit has chargeable and rechargeable first power supply and second power supply connected to a voltage transducer at input and output terminals, respectively, and a switch element connected between the output terminal of the voltage transducer and the second power supply. By controlling a rector current flown through a reactor connected to at least two switching elements at one end when the voltage transducer is started, a capacitor connected between the other end of the reactor and the ground is charged, and the switch element is brought in a connection state according to a charging voltage of the capacitor and a voltage value of the second power supply. Consequently, a vehicle power-supply unit can protect the elements by preventing a rush current during a backflow and reverse connection and at start-up.