PURPOSE: To make it possible to determine accurate particle size distribution by dispersing the same sample into two solvents having the different refractive indexes, measuring the angle distributions of the intensities of the scattered light beams from both solvents, and finding the refractive index of a particle.

CONSTITUTION: A sample W is uniformly dispersed into a solvent. The sample W is made to flow into a cell 1 under this state. A laser beam having the specified cross section is emitted from an emitting optical system comprising a laser light source 2 and a beam expander 3. The scattered light beams from the sample W are outputted through semiconductor photosensors P1 - Pn of a ring detector 5 and a photosensor Ps for measuring the sideward scattered light. The particle size distribution is outputted from a computor 12 based on the various kinds of relative refractive idexes for the scattering angle θof each photosensor and the memories of the theoritical scattered-light intensities of various kinds of particle diameters D. Thus, the accurate particle size distribu tion can be determined.