Luminous intensity and chromaticity of each luminescent light source are measured for an impressed current value, a mixing ratio of intensities of lights emitted from luminescent light sources required for synthesizing lighting intensity with the preset chromaticity at the luminous intensity instructed by an instruction value is computed with a theoretical equation for computing changes in chromaticity correlating to changes in luminous intensities of the luminescent light sources for color synthesis (ST33), luminous intensities of each luminous light source for generating a luminous intensity instructed by an instruction value from a mixing ratio are computed (ST34), and the impressed current value required for making each luminous light source emit light at the luminous intensity is read out from the characteristics of luminous light sources (ST35).