In an apparatus in which an electrode pair formed of two electrodes including multiple mutually parallel linear electrode pieces is provided in a container for storing particles dispersed movably in a medium to form an spatially regularly arranged electric field, the particles are migrated in the container due to the formation of the electric field by the application of a voltage to the electrode pair to generate a diffraction grating resulting from density distribution of the particles, diffracted light obtained by applying light to the diffraction grating is measured, and a particle size analysis or the like is performed from the temporal change in the diffracted light in the free diffusion process of the particles by stoppage or modulation of the application of the voltage, by making width L of the electrode pieces of the electrode pair and a space distance S between the electrode pieces as follows: L/(L+S)≦0.3, steep attenuation of the diffracted light intensity is not generated at the initial stage of the diffusion of the particles to accurately measure the particle size distribution.