An illuminance obtained by light passed through light diffusion type skylight windows is affected by the weather. This is to solve such problems by polarization of lighting devices and control technology in addition to improvements in the quality of skylights themselves, and the advantages of the high natural lighting offered by light diffusion type skylight windows are now being utilized. A set value that is the result of associating a required work surface illuminance for indoors with the work surface illuminance obtained by light passed through light diffusion type skylight windows and the generated power of a photovoltaic power generation panels is set as a reference for a light adjustment signal. In situations such as winter sunsets and rainy weather which are in a range equal to or less than the set value, light adjustment of lighting devices is performed by a signal transmitted from the photovoltaic power generation panels.