To provide a particle size measuring device that enables simple in-line measurement of the particle size even in a case of nano-sized particles during dispersion. Provided is a particle size measuring device which measures the particle size of particles that perform Brownian motion in a dispersion medium. The particle size measuring device includes a transparent column which accommodates a dispersion medium therein; a laser light irradiating unit which irradiates the dispersion medium in the column with laser light; an imaging unit which includes a camera that images the dispersion medium in the column; an image analyzing unit which acquires a displacement of corresponding particles from at least a plurality of images captured at a predetermined time interval Δt; and a calculating unit which calculates the particle size based on the fact that a root mean square value of the displacement is proportional to kBT/3πηd where kB represents a Boltzmann constant, T represents an absolute temperature, η represents a viscosity coefficient of the dispersion medium, and d represents the particle size.