A device for calculating diffraction efficiencies of a diffraction lens divided into a plurality of regions, each region comprising at least one grating ring, comprises a first memory for storing information about diffraction efficiencies of the regions; a second memory for storing information about weights corresponding to the regions; and a first processor for retrieving information from the first and the second memory, and calculating diffraction efficiencies of the entire diffraction lens using the formula E j = ∑ m = 1 M ⁢ W m ⁢ η mj ( 1 ) wherein: j: integer indicating the order of diffraction light Ej: diffraction efficiency for j-th order diffraction light of the diffraction lens M: positive integer (M>1) indicating the number of regions for which the diffraction efficiency is calculated m: index of the region for which the diffraction efficiency is calculated &eegr;mj: diffraction efficiency for the j-th order diffraction light of the m-th region (stored in the first memory) Wm: weight for the m-th region (stored in the second memory means). Thus, the diffraction efficiency of the diffraction lens can be calculated easily.