To solve such a problem that when solar radiation quantity is constant, output power from a solar battery changes like a P-I characteristic curve 31 according to output current, and when the extracted output current is IM, the maximum output PMat this solar radiation can be extracted, while the current is not necessarily controlled in such a manner conventionally, the extracted power was smaller than the maximum output PM, and the capability of the solar battery is not fully utilized.

A line obtained by connecting the maximum output points M of the P-I characteristic curves 31 for various solar radiation quantity is a maximum output point line 30. When current power generation state falls in a first region 32 where the maximum output point line 30 is larger than the P-I characteristic curve 31 (for example, Y point), the current is reduced to be close to the maximum output point M. Conversely, when it falls in a second region 33 where the line is smaller (for example, A point), the current is increased to be close to the maximum output point M. Thereby, the maximum output can be extracted. Even when the solar radiation quantity changes, the maximum output can be always obtained through the control as following a change in the solar radiation quantity.

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