PURPOSE: To restrict lowering of temperature from a set value after production of external disturbance to the minimum.

CONSTITUTION: There are provided an in-furnace temperatture sensor 2, an external temperature sensor 3, and an external disturbance detection temperature sensor 4 respectively in an electric furnace 1, on a furnace wall, and on a furnace inlet part. Cascade control is performed prior to application of the electric furnace, in which control detection temperatures by these temperature sensors 2 to 4 are compared and utilized. Hereby, temperature control pattern information required before the operation is obtained, which information is in turn stored in a memory device as a control parameter. An output of the electric furnace 1 is controlled using the control parameter stored upon the operation. Thus, there can be made minimum a time interval when in-furnace temperature and external temperature are unstable because a wafer is put into and out of the electric furnace.