A method of a particle size analysis in an optical measuring method, wherein the optical measuring method uses an apparatus comprising: a container that retains a gel sample or a liquid formed by dispersing particle groups movably in a medium, the container being made of a transparent material; a power supply that generates a voltage with a predetermined pattern including a direct current, frequency modulation and voltage modulation, or pattern capable of being arbitrarily set; an electrode pair provided in the container, which generates a regularly lined electric field distribution in the container by applying the voltage from the power supply; a control means that controls application of the voltage from the power supply to the electrode pair to control generation of a diffraction grating generated by density distribution of particle groups in the sample inside the container generated by dielectrophoretic force acting on the particle groups and disappearance thereof; a light source that irradiates beam of light to an area where the diffraction grating is generated inside the container; and a photo-detector that detects diffracted light of the beam of light by the diffraction grating, the optical measuring method performing a particle size analysis of the particle groups in the sample from a temporal variation of intensity of a diffracted light detected by the photo-detector, wherein the voltage applied to the electrode pair is stopped in a generated state of the diffraction grating and the temporal variation of the intensity of the diffracted light is measured in a disappearing process of the diffraction grating, wherein the method of analysis, comprises the step of: using the Einstein-Stokes relation, D = K B T / 3 ⁢ πηd, wherein K B is a Boltzmann constant as a diffusion coefficient, T is an absolute temperature of the particle-dispersed liquid to be measured, η is a viscosity of the particle-dispersed liquid or the gel to be measured, and d is a particle size of the particles to be measured, and an approximate analysis expression of a diffracted light attenuation, I t = ∝ exp - 2 ⁢ q 2 ⁢ Dt, which uses q = 2π / Λ defined by a particle concentration modulation period Λ in the diffraction grating generated by density distribution of the particle groups to obtain the particle diameter d. a container that retains a gel sample or a liquid formed by dispersing particle groups movably in a medium, the container being made of a transparent material; a power supply that generates a voltage with a predetermined pattern including a direct current, frequency modulation and voltage modulation, or pattern capable of being arbitrarily set; an electrode pair provided in the container, which generates a regularly lined electric field distribution in the container by applying the voltage from the power supply; a control means that controls application of the voltage from the power supply to the electrode pair to control generation of a diffraction grating generated by density distribution of particle groups in the sample inside the container generated by dielectrophoretic force acting on the particle groups and disappearance thereof; a light source that irradiates beam of light to an area where the diffraction grating is generated inside the container; and a photo-detector that detects diffracted light of the beam of light by the diffraction grating, the optical measuring method performing a particle size analysis of the particle groups in the sample from a temporal variation of intensity of a diffracted light detected by the photo-detector, wherein the voltage applied to the electrode pair is stopped in a generated state of the diffraction grating and the temporal variation of the intensity of the diffracted light is measured in a disappearing process of the diffraction grating, wherein the method of analysis, comprises the step of: using the Einstein-Stokes relation, D = K B T / 3 ⁢ πηd, wherein K B is a Boltzmann constant as a diffusion coefficient, T is an absolute temperature of the particle-dispersed liquid to be measured, η is a viscosity of the particle-dispersed liquid or the gel to be measured, and d is a particle size of the particles to be measured, and an approximate analysis expression of a diffracted light attenuation, I t = ∝ exp - 2 ⁢ q 2 ⁢ Dt, which uses q = 2π / Λ defined by a particle concentration modulation period Λ in the diffraction grating generated by density distribution of the particle groups to obtain the particle diameter d. using the Einstein-Stokes relation, D = K B T / 3 ⁢ πηd, wherein K B is a Boltzmann constant as a diffusion coefficient, T is an absolute temperature of the particle-dispersed liquid to be measured, η is a viscosity of the particle-dispersed liquid or the gel to be measured, and d is a particle size of the particles to be measured, and an approximate analysis expression of a diffracted light attenuation, I t = ∝ exp - 2 ⁢ q 2 ⁢ Dt, which uses q = 2π / Λ defined by a particle concentration modulation period Λ in the diffraction grating generated by density distribution of the particle groups to obtain the particle diameter d.