A phase distribution is calculated such that modulated light has a predetermined intensity distribution on a target plane and displayed on a phase modulation plane, readout light enters the phase modulation plane so as to generate the modulated light. When calculating the phase distribution, a region on the phase modulation plane is divided into N regions A1 . . . AN, with sizes set such that integration values of an intensity distribution in the regions are equal to each other. Further, a region on the target plane is divided into N regions B1 . . . BN, with sizes set such that integration values of an intensity distribution in the regions are equal to each other. The phase distribution is calculated by obtaining an optical path length from the region An to the region Bn, and determining the phase of the region An based on the optical path length.