In a spectrophotometer of the single-beam type, highly stable transmission and absorption spectra can be obtained with a high SNR while drifting is suppressed and for a long time even when the amount of light from the light source is varied over time. The spectrophotometer includes: a light source; a sample cell; a polychromator that generates a transmission spectrum of a sample in the sample cell by dispersing a portion of light from the light source that has passed through the sample into a plurality of spectral components; an image sensor that detects the transmission spectrum of the sample; a light source monitoring photodetector that detects a portion of the light from the light source that has not passed through the sample cell; and an operation unit that corrects the transmission spectrum of the sample by using an output signal from the light source monitoring photodetector.