To highly safely charge a main battery and an auxiliary battery by closing a switching means between the batteries when the terminal voltage of the main battery becomes higher than that of the auxiliary battery by a first set voltage or more, and also becomes higher than a second set voltage and opening the switching means when the terminal voltage of the main battery drops to a voltage higher than the first set voltage or the current value of the battery becomes higher than a set value.

A controller 7 is provided with a resistor 17 and a terminal voltage detecting means 19 which detects the main and auxiliary battery-side voltages of the resistor 17, and detects the magnitude of the current flowing toward an auxiliary battery 5 from a main battery 3 by comparing the main and auxiliary battery-side voltages of the resistor 17 with a third set voltage. When the terminal voltage of the main battery 3 is higher than that of the auxiliary battery 5 by a first set voltage or more and also higher than a second set voltage, a switching means 15 is closed and, when the terminal voltage of the main battery 3 drops to a voltage higher than the first set voltage or the current value of the battery 3 detected by means of a current detecting means 13 becomes higher than a prescribed set value, the switching means 15 is opened. Therefore, the batteries 3 and 5 can be charged highly safely.