A distribution of AC electric field regularly arranged in a cell is formed while storing a sample having particles dispersed in a medium in the cell, whereby the particles are dielectrically migrated in the medium to generate a diffraction grating by density distribution of the particles. Diffracted light generated by irradiating the diffraction grating by density distribution with measuring light is detected, and evaluation of dielectrophoretic intensities of the particles and/or the medium is performed from the detection result. According to this method, evaluation of dielectrophoretic characteristics can be performed without adhering a phosphor to particles, and since even a particle small in size can achieve a detection level by collecting a number of such particles to form a diffraction grating, dielectric characteristics of microparticles of several nanometers in diameter can be thus quantitatively measured with high sensitivity.