1. An optical-phase-distribution measuring method for measuring a phase distribution of a light to be measured at high speed and with high accuracy, said optical-phase-distribution measuring method comprising: an intensity-distribution measuring step in which an optical-phase-distribution measuring system comprised of two different optical systems with known optical characteristics (a first optical system and a second optical system) and at least a light-wave detecting sensor is used, said light to be measured is input to said first optical system and said second optical system respectively, intensity and phase of said light to be measured are modulated, said light to be measured which has been output is detected by said light-wave detecting sensor, and an intensity distribution of said light to be measured which has been detected is measured as an image; an observation-equation setting step in which based on said intensity distribution obtained at said intensity-distribution measuring step, optical characteristics of said first optical system, and optical characteristics of said second optical system, an observation equation of said optical-phase-distribution measuring system is set; a first nonlinear-optimization-problem formulating step in which from said observation equation, a phase-distribution identification inverse-problem by which said phase distribution of said light to be measured is identified is set, and said phase-distribution identification inverse-problem which has been set is formulated as a first nonlinear optimization problem in which a complex amplitude representing said light to be measured is assumed to be a design variable; a second nonlinear-optimization-problem formulating step in which by performing series expansion of said phase distribution of said light to be measured, said first nonlinear optimization problem is converted to a second nonlinear optimization problem in which expansion coefficients in a series expansion expression are assumed to be design variables; and a phase-distribution identifying step in which said phase distribution of said light to be measured is identified by solving said second nonlinear optimization problem.