A measuring method enabling simple and accurate measurement of a flow velocity distribution in a flow field inside a flow passage of an optical cell and a particle size-measuring method using the measuring method are provided. Providing a tracer particle of a smaller size than wavelength λ of laser light into the flow passage and capturing a bright spot attributed to light scattering from tracer particles by camera, and obtaining the flow velocity distribution by the analysis unit by obtaining an amount of movement of each tracer particle from movement of the bright spot and correcting a Brownian motion component from a correlation between an average value of variations of the amount of movement and Brownian motion are performed.