The purpose is to reduce the influence on the measurement due to high order diffracted light without arranging a filter for removing high order diffracted light between a diffraction grating and a PDA. The correction method includes a correction coefficient determination step of determining a correction coefficient related to a ratio of a portion of a detection signal value to the detection signal value, the portion of the detection signal value being derived from a second order diffracted light of light in the first wavelength range contained in the detection signal value of a long wavelength side photodiode for detecting light in the second wavelength range in the photodiode array, and a correction unit configured to obtain a corrected detection signal value derived from light in the second wavelength range from a different detection signal value of the long wavelength side photodiode by using the correction coefficient determined by the correction coefficient determination step.