PURPOSE: To improve accuracy by providing a photodiode and an optical filter for approximating the spectral sensitivity characteristics of the photodiode and a photoreceptor on the photodetecting side of said photodiode.

CONSTITUTION: A filter 8 is provided between a silicon photo diode 7 and a light source 4. If a blue filter having a peak of transmitted light wavelength at λ= 490mm is used as the filter 8, the spectral characteristics of the outputs of the respective photodiodes are approximated to the spectral sensitivity of the photoreceptor as shown in the graphs. The change with the color temp. of a light source of P(W)/G(W) in this case decreases particularly when the color temp. is high, and at T>2,500°K, the change is fairly approximate to 1 as shown. The output of the light intensity detecting sensor through the filter 8 is passed through an amplifier 9 and a comparator 10, by which a transistor 12 connected to a DC power source 11 is controlled to stabilize the quantity of light of the light source 4 automatically so that the light inducing a photoelectric effect under the same conditions is irradiated to a photoreceptor 1, whereby the reliability in the evaluation of the photoreceptor 1 is improved.