Abstract: In a spectrophotometer of the single wavelength-double beam type or the dual wavelength type, light signals of predetermined periods having an interval during which reference light serving as a reference enters, an interval during which sample light to be measured enters and an interval during which entering light is intercepted are detected and converted into electric signals by means of a photomultiplier. The electric signals are converted into digital signals by means of an A/D converter. Using the digital signals, a digital processing unit executes operations for measuring the transmission factor or the absorption of a sample and operations for changing the supplied voltage of the photomultiplier so as to keep the reference signal constant at all times. On the basis of a voltage operated in the digital processing unit, the supplied voltage of the photomultiplier is changed in the interval during which the entering light is intercepted, whereby the stability of the feedback system is enhanced.