An object is heated to a preheating temperature in an atmosphere of a reducing gas under the atmospheric pressure while adjusting the setting of the emissivity of a non-contact temperature measuring part and regulating the temperature of the object according to the measured value measured by a contact temperature measuring part. The pressure of the atmosphere is reduced. The object is further heated to a heating temperature under a lowered pressure while regulating the temperature of the object according to the measured value measured by the non-contact temperature measuring part whose setting of the emissivity is adjusted during the heating process to the preheating temperature. The pressure of the atmosphere is increased back to the atmospheric pressure while maintaining the heating temperature of the object. The temperature of the object is decreased under the atmospheric pressure. With this, in the process of heating an object under a lowered pressure, the actual temperature of the object is managed over the whole steps, and the object can be most suitably heated according to the actual temperature.