In a wavelength displacement correcting system and method of the invention, a monochromatic beam from a light emitting diode driven by a constant current is incident through an incident slit of a spectral device. The incident beam is diffracted on a diffraction grating to form a dispersed light image, and information relating to the dispersed light image of the incident slit is outputted from the spectral device. A wavelength displacement amount of the monochromatic beam is calculated, using a forward voltage value corresponding to the constant current, and a forward voltage initial value. Wavelength displacement amounts of at least two diffracted beams are calculated, using output values of the at least two diffracted beams, and diffracted beam output initial values with respect to the dispersed light image. A dispersion width displacement amount of the spectral device is calculated, using the calculated wavelength displacement amount of the monochromatic beam, and the calculated wavelength displacement amounts of at least two diffracted beams, whereby a wavelength displacement of the spectral device is corrected.