PURPOSE: To speedily transfer to proper maximum electric power follow-up control when the sunshine increases thereafter even if the output voltage of a solar battery drops at low-sunshine time.

CONSTITUTION: A command value voltage Vset which increases in steps of width Vstep is added to the control signal of an inverter, the current output electric power Pnow of the solar battery is compared with the value obtained by increasing or decreasing last output electric power Ppast by set electric power Phist where a dead zone is set, and the command value voltages Vpast and Vnow are increased or decreased in steps of width Vstep according to the comparison result to set a next-time command value voltage; when the output electric power is in the dead zone, the command value voltage Vnot is forcibly decreased with the step width Vstep to set a next-time command value voltage and the inverter is driven under maximum electric power follow-up control. If the output electric power of the solar battery drops below a set value Pmin at low-sunshine time, a voltage at which the inverter can be driven is regarded as a lower limit of operation irrelevantly to the last output electric power Ppast and the output voltage Vcell of the solar battery is lowered to drive the inverter continuously.