To measure the amount of incident light accurately by highly accurately estimating the average value of the number of photoelectrons discharged from the photoelectric conversion surface of a photodetector.

When an incident luminous flux A is incident on a photodetector 10, photoelectrons are emitted and amplified, and the electric signal is outputted. This electric signal undergoes integration for a specified time by an integrator 20 and becomes the voltage signal. The voltage signal is converted into the digital value by an AD converter 50. The digital value is inputted into histo-memory 60, and the crest-value distribution of the voltage signal is formed. Then, in an operating part 70, the distribution of the number of the photoelectrons, which are emitted from the photodetector 10 on which luminous flux to be measured is incident, is estimated in a photoelectron-number-distribution estimating means 73 based on the crest-value distribution formed at the incidence of the luminous flux to be measured in a photodetector 10, the crest-value distribution of the single photoelectron phenomenon obtained in a crest-value-distribution forming means 71 for the single photoelectron phenomenon and the crest-value distribution of k-photoelectron phenomenon (k=1, 2, 3,...) computed in a crest-value-distribution forming means 72 for the k-photoelectron phenomenon. Furthermore, the amount of the light of the luminous flux to be measured is obtained.