To provide a device for measuring a particle size having a particle dispersion part for dispersing aggregates of powder particles as single particles effectively and uniformly into fluid, capable of miniaturizing the particle dispersion part, reducing its cost, and improving measurement accuracy of particle size.

The device for measuring the particle size has a particle size measuring part; a particle supply tube for supplying particles to the particle size measuring part; and the particle dispersion part 4 connected to the particle supply tube, for dispersing the particles supplied to the particle size measuring part uniformly into a fluid medium. The particle dispersion part has dispersion containers 41, 42 wherein a dispersion chamber 40 which is a cylindrical-shaped space is formed inside, equipped with a particle inflow port 421 communicated with a periphery of the center axis of the dispersion chamber and a particle outflow port communicated with a peripheral part of the dispersion chamber; a rotor plate 43 provided rotatably inside the dispersion containers in the state centered at the center axis; a rotation driving part 44 for performing rotation driving of the rotor plate; and a plurality of rotating projection parts 431 provided protrusively in the center axis direction on the surface of the rotor plate.