In the present invention, first boosting circuits (41a to 41d) are interposed upon each of direct current power lines (La to Ld). The boosting ratios of the first boosting circuits (41a to 41d), for each iteration of a first cycle, are variably controlled during a first period so that the generated power of the corresponding solar cell strings (1a to 1d) is maximized, and the boosting ratios during a second period are controlled so as to be maintained at a uniform value. The total amount of time of the first period and the second period is made to correspond to the first cycle.