Optical spectrum of a light irradiated on the object, a spontaneous light emitted from within the object or a surface thereof, a scattered light, a transmitted light, a reflected light or a refracted light from within the object or a surface thereof generated by irradiating light on the object is resolved by amplifying said lights in a bandwidth narrower than the bandwidth of the optical spectrum of said lights. Thus, a method for detecting optical spectrum is provided in which signal intensity is amplified while deterioration of spectral data is suppressed.
