PURPOSE: To supply material to be treated into a crucible correctly and prevent the overflow of treating liquid from the crucible surely by a method wherein the position of the upper surface of the material to be treated, which is supplied into the crucible, is detected to stop a material to be treated supplying means based on the detecting signal.

CONSTITUTION: The tip end of a pipe 5 is projected into a crucible 1 by a cylinder 52 to supply nitrogen gas into the curcible 1 from a gas supplying source 64 through a pipeline 63 while gas is supplied from a gas supplying source 64 into a covering cylinder 54 through a pipeline 63. Melting material and the like are supplied from a screw feeder 8 while rotating the crucible 1 about the shaft 10 thereof while liquid 11 to be treated is supplied into the crucible 1 through another pipeline 65 by driving a pump 66. Further, microwave is projected through a wave guide 9 to heat and melt the material in the crucible 1. When the level 111 of the liquid 11 is elevated to the tip end of the pipe 5 and the back pressure in the pipeline 55 is elevated, it is detected by a back pressure detector 57 and the pump 66 is stopped by a controller 50.