To provide a crystal grain size controlling unit for crystallizer with which crystals having uniform grain diameters, namely, having a unimodal grain size distribution curve can be obtained.

This crystal particle size controlling unit is provided with a rotational speed-variable agitation means 2 arranged in a crystallizing part 1 of the crystallizer, a turbidity detecting means 4 which is arranged in the part 1 for detecting the turbidity of a solution in the part 1 and consists of an optical sensor, an electric conductivity detecting means 5 arranged in the part 1 and a control means 3 for inputting the signals of the information on the turbidity and concentration of the solution from the means 5 and controlling the rotational speed and agitating time period of the means 2 according to the prescribed control program.