A measurement method and an evaluating apparatus are provided which accurately evaluate the light amount of a spot beam, the diffraction efficiency, and the intensity distribution in the optical axis direction by detecting even a weak diffracted beam in an arbitrary wavelength range converged by a diffraction optical element as an imaging lens. Light emitted from a white light source passes through a wavelength band-pass filter and is diaphragmed by a pinhole slit. The resultant light is paralleled by a collimator lens and enters a diffraction optical element as an imaging lens. The light getting out from the diffraction optical element is converged to be a spot beam, is magnified by a microscope 18, and is then projected on a CCD. A distance changing member changes the distance between the CCD and the diffraction optical element, and then, the intensity distribution in the optical axis direction is measured.