PURPOSE: To produce a high-quality crystal without any flaw in spite of its rapid growth by controlling the velocity of an upward current on the outside of a draft tube.
CONSTITUTION: When an impeller 18 is rotated in the crystallizer 1 provided with the draft tube 4 and a baffle plate 7, an downward current is formed on the inside of the draft tube 4, an upward current is generated on the outside, and a circulating current is formed between the inside and outside of the draft tube 4. The flow velocity of the upward current on the outside of the draft tube 4 is sufficiently decreased by adjusting the r.p.m. of the impeller 18 and the pitch angle of the impeller, hence the crystal of the size visible from the outside and floated up by the upward current to some extent does not overflow the upper end of the draft tube 4, and the crystal is not circulated along with the upward current. As a result, a high-quality crystal without any flaw can be produced in spite of its rapid growth.