PURPOSE: To reduce scaling, by moving a set ring region into a crystallizing canister to avoid cooling due to the air.

CONSTITUTION: A buffle plate 7 surrounds the outer periphery of a draft tube 4 in such a manner that a set ring region 6 is formed in the crystallizing canister 1 remote from the canister wall thereof, that is, just to the outer periphery of the draft tube 4 and the upper part thereof is closed while the lower part thereof is opened. By moving the set ring region 6 into the crystallizing canister 1, cooling due to the air is eliminated and scaling is extremely reduced. In this case, a mechanism wherein fine crystals are withdrawn from the set ring region 6 to be supplied to a heater 2 through a recirculation pump 10 and dissolved under heating to be imparted to the crystal stayed in the canister 1 as oversaturation is similar to that of a conventional canister.