Provided is a spectroscopic detector including: a light source for generating polychromatic light 11; a single diffraction grating 13; an excitation optical system for guiding the light from the light source 11 onto the diffraction grating 13, for selecting one wavelength from the light diffracted by the diffraction grating 13, and for casting the selected wavelength of light into a sample as excitation light; a detection optical system for guiding observation light emitted from the sample irradiated with the excitation light onto the diffraction grating 13 to disperse the observation light; and a photodetector 15 for detecting the observation light dispersed by the detection optical system. By using one diffraction grating 13 in both the excitation optical system and the detection optical system, the number of diffraction gratings is reduced, whereby both the cost reduction and the downsizing of the device are achieved.