A spectroscopic detection device including: a stop in which an aperture is formed; a first and second photodetectors which detect detection light; a collimator which converts the detection light emitted from the stop into substantially parallel light, and emits light to at least one of the first and second photodetectors; a dispersive element which is arranged between the collimator and the first photodetector, and disperses the detection light; a condensing optical system which condenses the detection light dispersed by the dispersive element to the first photodetector; and a wavelength selection filter which is arranged between the collimator and the second photodetector, and allows light in a specified wavelength range to enter the second photodetector. The collimator is configured so that the focal length for the detection light emitted to the first photodetector may be different from the focal length for the detection light emitted to the second photodetector.