The problem of retrieving the particle size distribution is solved by using the optical diffraction method. Angular distribution of scattered laser beam (670 nm) is measured by ring-gap photodetector movable along the laser beam. The elimination of undesirable optic reflections using lens-less concept, keeping the geometry of ring-gap diaphragm as well as integral power spectrum measurement stable, and the possibility of measuring relatively large depths of scene are the main advantages of such a measuring scheme. The numerical solution of this ill-posed inverse problem in small scattering angle approximation is based on regularization method, minimizing the Tikhonov＇s functional. It is shown that the inverse problem of separation of particle size fractions dispersed in a deep volume is solvable with reasonable accuracy maintaining the stability of the solution.