A plurality of individually corresponding heaters and temperature sensors are provided along with a plurality of a PID control means. An average temperature/gradient temperature calculating means calculates the average temperatures of the temperatures measured by the temperature sensors and the gradient temperatures based on the measured temperatures. Each of the PID control means outputs an operation signal so that the average temperatures or gradient temperatures may be made equal to the desired values. A distributing means distributes the operation signal from each of the PID control means to each heater so that control by the PID control means may produce no effect on the control of the other PID control means.