To reach a maximum power point in a short time, to eliminate the deviation from the maximum power point, and to eliminate the stop of an inverter in the case of abrupt variation in the quantity of solar radiation by reducing the variation width of a target solar battery operating voltage each time the direction of varying the target solar battery operating voltage is inverted.

When current solar battery output power is larger than last solar battery output power in case of variation in solar battery operating voltage with the target solar battery operating voltage in every specific time, the direction of the variation of the target solar battery operating voltage is set to the same direction and when smaller, the direction of the variation is inverted; and the variation width of the target solar battery operating voltage is made small each time the direction of the variation in inverted. Further, the variation width of the target solar battery operating voltage is made large if the current solar battery output power varies by more than a specific value from the last solar battery output power when the solar battery operating voltage varies as the target solar battery operating voltage in every specific time varies.