PURPOSE: To provide a high accuracy temperature controling method in the control of temperature in a diffusion furnace for semiconductor production by a method wherein thermocouples are installed at the heater around furnace core pipe and at a point near the core pipe, with detected temperatures and their difference being used to calculate core pipe temperature.

CONSTITUTION: A first thermocouple 4 is installed at B area of cylindrical resistance heater 2 surrounding furnace core pipe 1. A second thermocouple 5 is provided at C area which is close to the core pipe 1. Temperatures T1 and T2 detected by both thermocouples are sent to the temperature controller 9, which calculates the temperature in the core pipe, from the radial temperature gradient including the heater and pipe, T1-T2, and either T1 or T2. The power supply 7 is controlled to supply power to the heater 2 so that the difference between the calculated temperature and the set value for the furnace becomes zero in accordance with the required response characteristic.