A device which computes an angular range of illumination of an electron beam in which aberrations in an optical system can be measured efficiently by a tableau method. The device (100) includes an aberration coefficient information acquisition portion (112) for obtaining information about aberration coefficients of the optical system, a phase distribution computing portion (114) for finding a distribution of phases in the electron beam passed through the optical system on the basis of the information about the aberration coefficients, and an angular range computing portion (116) for finding the angular range of illumination on the basis of the distribution of phases found by the phase distribution computing portion (114).