PURPOSE: To prevent the occurrence of oxidation and carburizing in the high temp. heating zone in the periphery of a burning flame and in the carburizing completion part.

CONSTITUTION: A work W as a material to be treated is provided to the inside of a chamber 1. Then, acetylene gas, not lower than the equivalent ratio to oxygen, is supplied into the chamber 1 and burned and, by the combustion of the acetylene gas of not lower than the equivalent ratio, the oxygen in the air in the chamber is removed and also the burning flame of the acetylene gas is brought into contact with the surface of the work W through a nozzle 2 to apply heating, by which carburizing treatment is done.