PURPOSE: To accurately control a temperature by switching to a closed loop control after a difference of gradients of a measured temperature and a target temperature control pattern, is reduced by certain degree even if it falls within a closed loop controllable temperature range.

CONSTITUTION: After heating, an open loop control for controlling outputs of lamp groups 6 is executed according to a predetermined lamp output control pattern until an output of a radiation thermometer 12 reaches a predetermined closed loop controllable temperature range. When it reaches the closed loop controllable temperature range, a ratio of a gradient of a measured temperature by the thermometer 12 to that of a target temperature control pattern, is obtained. The open loop control is continued in the meantime while the temperature gradient ratio is smaller than a predetermined control switching reference value, and a closed loop control for controlling a lamp output so that the measured temperature coincides with the target temperature control pattern if it becomes a reference value or more. A lower limit of the closed loop controllable range is a lowest temperature or higher to be measured by a radiation thermometer, and set, for example, to 300°C.