To avoid the breakage of an insulating tube which generates a plasma flame by preventing the adhesion of contaminants to the tube during shutdowns.

An inductively coupled plasma heating furnace generates high-frequency plasma by introducing a working gas from a gas supply system 2 into a cylindrical insulating tube composed of inner and outer insulating tubes 1 and 7 and impressing high-frequency power obtained by making a high-frequency current flow to an annular coil 3 from a high-frequency power source 4 upon the working gas and heats and melts an object to be treated arranged in the lower furnace casing 10 of the furnace by irradiating the object with the plasma. When the furnace is shut down, the supply of the high-frequency power and introduction of the working gas are stopped and, successively, a helium gas is supplied into the inner insulating tube 1 from a gas supplying system 2 until the tube 1 is filled up with the helium gas.