PURPOSE: To convert the structure of an apparatus to such a structure that a particle size to be withdrawn can be easily selected and to make the same simple and inexpensive, by assembling a solid-liquid separator utilizing a screen in place of a set ring region.

CONSTITUTION: A solid-liquid separator 17 utilizing a screen 18 attached to the canister wall of a crystallizing canister 1 provided to a draft tube 4 so as to leave a proper distance between said canister 1 and a draft tube 4 is assembled in place of a conventional set ring region. In this structure, a particle size to be withdrawn is determined by the mesh of the screen 18 while fine particles with a particle size of a predetermined value or less is sucked from the solid-liquid separator 17 through a recirculation pipe 16 by a recirculation pump 10 to be sent to a heater 2 under pressure and dissolved herein to be imparted to the crystal stayed in the crystallizing canister 1 as oversaturation. Therefore, the selection of the particle size to be wrthdrawn is easy while the crystallizing canister can be miniaturized and, therefore, installation cost can be reduced.