To provide a grain size distribution-measuring apparatus for accurately measuring a grain size distribution.

A grain size distribution-measuring apparatus D comprises a circulation channel 1 for circulating a sample solution 4 where sample particles 2 are dispersed into a dispersion medium 3, a pump 5 for circulating the sample solution 4 in the circulation channel 1, and a flow cell 6 that is provided in the circulation channel 1 for measuring the grain size distribution of a sample particle 2 in the sample solution 4. In the grain size distribution- measuring apparatus D, the circulation change in the sample solution 4 in the circulation channel 1 is changed by controlling the pump 5.