PURPOSE: To monitor an aging decrease in the irradiance of an ultraviolet part and the lighting condition of a lamp, by measuring the irradiance of the ultraviolet and that of a visible part through ≥2 photosensors in a metallic block case controlled at constant temperature.

CONSTITUTION: While ultraviolet rays with a specific wavelength from a light source is made incident to a photosensor 4 in the metallic block 7 controlled at the constant temperature through a diffusing plate 1, interference filter 2, and light guide 2, visible light with a specific wavelength is made incident to a photosensor 4'. Those incident light beams are converted by current-voltage converters 18 and 18' into electric signals, which are further converted by voltage-frequency converters 12 and 12' into frequencies, thereby displaying the irradiances on display devices 15 and 15' through arithmetic circuits 13 and 13' and integrating circuits 14 and 14'. A measurement of the visible part is utilized to monitor the lighting condition of the lamp, and a measurement of the ultraviolet part is utilized to monitor the deterioration of the lamp and the photodetection amount of a sample.