A spectrometry device includes a spectroscope, an extraneous light sensor, and a light intensity controller. The spectroscope includes a light source that emits illumination light to a medium and a wavelength-selective interference filter that performs spectroscopy on light incident from the medium. The extraneous light sensor detects the intensity of extraneous light which is incident on the medium. The light intensity controller controls the intensity of the illumination light emitted from the light source such that the light intensity ratio of the illumination light and the extraneous light is equal to a first value.