<p>Under a state where a sample having a group of particles to be measured dispersed movably into a medium is stored in a sample cell, the cell is irradiated with ultrasonic wave to generate a diffraction grating by density distribution of the particle group and diffraction light generated by irradiating the diffraction grating with a parallel light beam is detected. After a diffraction grating is generated by irradiating ultrasonic wave, the particle group is diffused by stopping irradiation of ultrasonic wave to extinguish the diffraction grating, and then the particle diameter is obtained by obtaining a diffusion coefficient from diffraction light intensity change with time in the extinction process of the diffraction grating. Since the diffraction grating is generated by the density distribution of particle group using ultrasonic wave, even such particles as dielectrophoretic power does not act can be measured without being affected by the diffraction light due to an electrode pair when the dielectrophoretic power is utilized.</p>
