To raise the temperature of an article to be heat treated gently and quickly up to a highest target temperature by providing a preheating chamber at the predriving part of a heating chamber, heating the low temperature atmosphere in the preheating chamber by feeding it into the high temperature heating chamber by means of a fan and then circulating the heated atmosphere back to the preheating chamber.

Combustion radiation pipes 2 are fixed to the ceiling part and the bottom part of a heating chamber 1, a preheating chamber 3 is disposed at the predriving section of the heating chamber 1 and a fan 5 is secured above the inlet 4 of the preheating chamber 3. Atmosphere is sucked from the preheating chamber 3 by operating the fan 5 and fed through an atmosphere moving passage 6 into a part of the heating chamber 1 before being returned back to the preheating chamber 3. A conveyor belt 8 is disposed in the furnace and a thermocouple 10 disposed closely to the conveyor belt 8 substantially in the center of the heating chamber 1 is connected to a combustion control circuit for the combustion radiation pipes 2 in order to keep the temperature of the heating chamber 1 at a desired level. Further, a thermocouple 11 is provided above the central part of the preheating chamber 3 and connected with a rotation control circuit for the fan 5 in order to keep the temperature of the preheating chamber 3 at a desired level.