Provided is a method for performing a wavelength calibration of a monochromator with a diffraction grating by casting light from a standard light source whose emission intensity contains a change with a predetermined cycle onto the diffraction grating and measuring an intensity of light reflected by the grating. The method includes the steps of: measuring at least two times the intensity of the reflected light from the grating within the aforementioned cycle at each of the rotational positions of the grating corresponding to a range of wavelengths including a peak wavelength of a bright line spectral light generated by the standard light source; determining an intensity value 201 at each rotational position based on all the measured values obtained at the rotational position; and locating, as the peak wavelength of the bright line spectral light, a wavelength at which the intensity value 201 is maximized.