To restrain damage to a minimum by quickly detecting such trouble as flow-out of molten metal caused by breakage, etc., of a molten metal supplying tube.

Surrounding of a ceramic tube 15 as the molten metal supplying tube 6 is covered with a heat-insulating material 16, and on the upper part of the ceramic tube 15, bar-like heaters 17 are disposed and also, on the lower part of the ceramic tube 15, a molten metal leakage sensor 18 is disposed. Further, in the molten metal leakage sensor 18, when the molten metal leakage is detected, an interlock circuit, with which the molten metal in the center part of the ceramic tube 15 is returned back to a molten metal supplying furnace 5, is connected by working an air cylinder 11 in a pulse-rod control so as to make the inner part of a furnace to the negative pressure.