Provided is a spectrophotometer having a sample container 30, a light-source unit 10 for casting measurement light into the sample container 30, a photodetector 40 for detecting light obtained from the sample container 30 illuminated with the measurement light, a light separator 20 placed between the light-source unit 10 and the sample container 30, an A/D converter 50 for converting detection signals from the photodetector 40 into digital signals, and an A/D conversion time controller 65 for controlling an A/D conversion time in the A/D converter 50. The A/D converter 50 receives, for each A/D conversion time, detection signals sequentially produced by the photodetector 40, and sequentially outputs values corresponding to the amounts of signals received. The A/D conversion time controller 65 controls the A/D conversion time at five times (preferably, ten times) the cycle of commercial power supplies or longer during wavelength-correctness validation of the light separator 20.