PURPOSE: To realize real time control to make the agreement between the particle size distribution of a group of particles formed and the desired value in a particle treating plant for forming liquid crystal particles in PDLC, etc.

CONSTITUTION: A desired value generating means (a) for converting the particle size distribution desired value into a diffracted/scattered light intensity distribution desired value, a irradiating optical system (c) for irradiating laser rays to a group of particles in a particle treating plant (b), an intensity distribution measuring optical system (d) for the diffracted/scattered light obtained by the irradiation and a control signal generating means (e) for supplying a control signal to the particle treating plant (b) based on the output of the desired value generating means (a) and the light intensity distribution measuring optical system (d) are provided.