comparatively large amount from the surface of the melting solution the metal fluoride which is volatilized is moved and diffused to various places in the apparatus for pulling the single crystal and is condensed and solidified in contact with a low temperature member in the apparatus it is supposed that the metal fluoride which is solidified is dropped for some reason and is also dropped and mixed into the melting solution in the crucible depending on circumstances it is supposed that the melting solution partially causes a reduction in a temperature and an impurity is often brought from the outside of the crucible when such a solidified matter is mixed into the melting solution furthermore it is supposed that a physical disorder wave on the melting solution surface is generated due to a shock caused by the drop of the solidified matter it is supposed that these phenomena adversely influence the growth of the single crystal and the crystal is thus cut in a middle or bubbles are generated resulting in the difficulty of a stable crystal growth in order to lessen the volatilization of the metal fluoride from the melting solution surface it is preferable to reduce the interval between the internal surface of the sidewall of the outer crucible and the external surface of the sidewall of the inner crucible thereby decreasing a surface area of the melting solution in consideration of the effect of reducing the solid impurity contained in the melting solution as described above however it is preferable that the interval between the internal surface of the.