PURPOSE: To perform measurement of spectral sensitivity in a short time, by detecting the light quantity of the one hand light having a different wavelength by a light quantity detector for measuring the spectral sensitivity and projecting the other hand light to an optical sensor.

CONSTITUTION: Outputs of a light quantity detector 8 and a sample 9 are obtained under a dark condition by closing an optical path with an optical path shutter 4 and next, the outputs of the sample 9 and the detector are obtained by opening the optical path and controlling the light quantity at every wavelength. Further, the surface light quantity of the sample is obtained by subtracting the output at the time of the dark condition and also, multiplying a correction factor for obraining the surface light quantity of the sample by the output of the detector 8 and spectral sensitivity is obtained from sample output/light quantity. This process is repeated at every each wavelength. Hereby, measurement of the spectral sensitivity is measured in a short time because both outputs of the detector 8 and the sensor 9 are detected at the same time.