PURPOSE: To effectively utilize the output of a solar cell by controlling a contactor providing between the cell and a storage battery for preventing them from overcharging by a voltage monitor between the cell and a load.

CONSTITUTION: The output of a solar cell 1 is input from a connecting point 6 through a reverse current preventing circuit 3 to a DC/AC converter 4 which outputs an AC, thereby driving a load 5. The point 6 is connected through a contactor 8 to a storage battery 7. The voltage VX of the point 6 is compared by first and second comparators 11, 12 with an overcharging level VH and a reset level VL. When the voltage VX of the point 6 becomes VX>VH, the output of the comparator 11 turns ON a contact 17, a trip coil 16 turns OFF the contactor 8, while when the voltage VX becomes VX<VL, it turns ON a contact 19, and an energizing coil 18 turns ON the contactor 8. Thus, the output of the cell 1 is effectively utilized thereby to prevent the battery 7 from overcharging.