This device allows the determination of the mean size of particles in suspension or of droplets of an aerosol. Particles pass through a laser beam that is scattered following an angle which is inversely proportional to the particle size. Each particle causes the deviation of a part of the incident beam and scattered light is focalized onto a detector consisting in concentrical rings. The signal intensity received by each ring is proportional to the number of particles having such a size that the beam was deviated to that particular ring. The signal is treated by a computer that gives a volume distribution of the particle size.