In this paper, a set of equations for retrieving phase from two intensity distributions in the image plane or some other plane of an imaging system was obtained by the exactly mathematical derivation. The equations are completely analogous to the algorithm suggested by Gerchberg-Saxton[1,2,4] and Misell [3]. We point out that, if the transformation between two wave functions is performed by unitary operator and one of the wave functions is absolutely square integrable, the phase reconstruction from the known intensity distributions in any two planes of an imaging system can be achieved using OS algorithm.