To provide a technique of automatically and continuously estimating the parameters for particle distribution such as the average particle diameter of a crystal to be produced in a crystallization can, the ratio of existing microcrystals, and the existence ratio of existing large crystals at high precision and to provide an economical apparatus for the estimation.

While time-series data of the quantity of transmitted light and the quantities of forth and back scattered light is obtained a particle state detector 6 by radiating light to a slurry solution taken out by a product taking pump in a crystallization apparatus using a crystallization can 1, the data of slurry concentrations in the crystallization can and a circulation line. The parameters for particle diameter distribution are estimated by carrying out computation using (either totally or partly) the above defined data and also various kinds of data such as the rotation speed of a stirring apparatus 9, the current value of a circulation pump 4, the input and output temperature of a heater in the circulation line, the pressure in the gas phase in the crystallization can, etc., as process data relevant to the crystallization can.