A reference light source produces a reference light having a predetermined light intensity. A sensitivity measuring device acquires a detection signal from an operation, in which the reference light having been produced by the reference light source is photoelectrically detected with the photomultiplier. The sensitivity measuring device measures a sensitivity of the photomultiplier in accordance with a relationship between the thus acquired detection signal and a reference signal, which has been set previously and which has a level in accordance with the predetermined light intensity of the reference light.