To obtain an accurate photometric value by modifying the applying voltage when a photomultiplier is receiving a light of different quantity to acquire information indicative of the relation between the applying voltage and the sensitivity of the photomultiplier and then utilizing that information.

The photometer 2 comprises a light source unit 21 comprising a light source 24 and a turret plate 25 having a plurality of aperture stops and connected with a controller, and a measuring head 22 comprising a photomultiplier 27 and an applying voltage supply circuit 29. The quantity of light for irradiating the photomultiplier 27 can be varied by turning the turret plate 25. Upon ending a predetermined control flow, the controller determines variation in the sensitivity of the photomultiplier 27 for each applying voltage and stores the variation in a memory. When photometry is performed, the light source 24 is removed from the optical path and the aperture stops are arranged on the optical path in order to detect the light from a sample by means of the photomultiplier 27. The controller obtains the information of voltage currently applied to the photomultiplier 27 and determines the current sensitivity based on a data from the memory and then calculates a photometric value based on a signal from the photomultiplier 27 and the sensitivity.