To provide a spectrophotometer with a diffraction grating vibrating quickly.

In a spectrophotometer, a light source is constituted in a form of a plurality of light emitting diodes for transmitting light falling on a vibrating diffraction grating 13 via an entrance slit 14, and the light is dispersed by the diffraction grating 13 toward an outlet slit 16 while being shed on a sample via a narrow light band width. The wavelength propagated via the outlet slit 16 is scanned over a selected spectrum buy the vibration of the diffraction grating 13. A diode array 17 emits light in each different wavelength band, and one group of diodes emitting spectrum are continuously excited or turned off when the diffraction grating 13 vibrates, thus exciting only two diodes for a specific moment.