PURPOSE: To eliminate the affection of the convection of pressure gas in a heating chamber effectively and permit the holding of temperatures at the upper and lower parts in the heating chamber uniformly at all times by a method wherein the outputs of respective heating sources are unified when a pressure in a furnace is vacuum or normal pressure while the output of the heat source at the side of the lower part is increased to obtain a specified output rate when the pressure in the furnace is high.

CONSTITUTION: When a pressure in a furnace is vacuum or normal pressure, the outputs of respective heaters 5, 6 are equalized substantially so that the ratio of respective outputs becomes 50% of the whole output respectively. According to this method, a work zone 4 is heated uniformly from the surroundings thereof and a work in the work zone 4 may be heated uniformly from the upper and lower sides thereof. On the other hand, when the pressure in the furnace is increased, the output of the lower heater 6 is increased. When the pressure in the furnace is increased to a degree higher than 100kg/cm2, the output of the lower heater 6 is increased to the degree of 85% of the whole of outputs while the output of the upper heater 5 is decreased to the degree of 15% of the whole of outputs. According to this method, the affection of the convection of pressure gas, which is generated when the pressure in the furnace has become high, may be eliminated and a temperature difference between the upper part and the lower part of a heating chamber 3 may be reduced sufficiently.