To provide a melting furnace which can efficiently perform mass treatment without causing contamination in the furnace.

The melting furnace is constituted in such a way that a heat insulation material 2 and an inside-furnace heating heater 3 to surround a treatment space S and, at the same time, a pipe-like gas heating heater 6 for heating a supplied gas X to be introduced to the space S is provided in the furnace and the furnace melts a sample (a) placed in the space S by blowing the supplied gas X which is heated while the gas X is made to flow though the heater 6 against the sample (a). Therefore, such narrowing down as that made to form a plasma jet becomes unnecessary and stable gas supply becomes possible even in a pressurized atmosphere.