A spectral detector includes a plurality of spectral detection units, each of the spectral detection units including an optical signal processor configured to deliver an optical signal incident to the spectral detection unit to an outside of the spectral detection unit, and a resonator configured to modulate a spectrum of an optical signal incident to the optical signal processor by interacting with the optical signal processor, at least some of the resonators of the plurality of spectral detection units having different lengths from each other, and a number of optical signal processors included in each respective spectral detection unit varying according to a length of the resonator included in the respective spectral detection unit.