A provided optical-phase-distribution measuring method, by which optical phase distribution is identified at high speed and with high accuracy from information on light-intensity distribution without using a special measuring device, comprises steps: for inputting light to be measured to optical systems, respectively, modulating the intensity and the phase, detecting the output light to be measured with CCD, and measuring the intensity distribution of detected light to be measured as an image with an optical-phase-distribution measuring system provided with the two different optical systems; for setting an observation equation, based on the intensity distribution and on the optical characteristics of the optical systems; for setting a phase-distribution identification inverse-problem from the observation equation, and formulating the set phase-distribution identification inverse-problem as a first nonlinear optimization problem in which complex amplitude representing the light to be measured is assumed to be a design variable; for converting the first nonlinear optimization problem to a second nonlinear optimization problem, in which expansion coefficients in a series expansion are assumed to be design variables, by series expansion of the phase distribution of the light to be measured; and for identifying the phase distribution of the light to be measured by solving the second nonlinear optimization problem.