A laser beam emitted from a laser source is split by a beam-splitting means such as a beam sampler, and the power Q of the split beam is measured by a first detector. In addition, the power q1 of light that has passed through a pinhole while a DOE is not set is measured by a second detector, and the power ratio α=q1/Q is calculated. Then, the DOE is set and the power ratio βk=qk/Q, where qk is the power of each light beam, is calculated. The power ratio βk is evaluated on the basis of the power ratio α, so the optical properties of a diffractive optical element, in particular, in terms of diffraction efficiency in laser-beam diffraction and intensity uniformity of split beams can be measured with high accuracy.