The Ishikawa mixing and grinding machine crushes large particles into small ones. To check the progress of mixing and grinding, the particle size monitoring is very important. Recently, we proposed the progress investigation of the Ishikawa mixing and grinding machine by sound signal processing successfully. Averaged Power Spectrum Density(PSD) values processed by measured sound signal are employed to estimate the progress remotely. It is found that PSD value also decreases as particle size becomes smaller by grinding. However, the particle size is only observed roughly by pictures. In this paper, we study the correlation between the particle size distribution of material and PSD. The particle size distribution is obtained by imaging analysis with micro scope. As a result, the correlation coefficient between particle size and sound signal is beyond 0.7.