PURPOSE: To control the temperature of reactor exactly and economically by measuring the inlet temperature, flow rate, and outlet temperature of a heat exchanger for a heating medium circulating in the heat exchanger to obtain the heat exchanging load and giving given load elements to the controlled amount of the heating medium of a controlling device for the addition and flow rate control of the heating medium.

CONSTITUTION: The inlet temperature of the heating medium 5 circulating in the heat exchanger 3 is detected by the detector 8, the inlet flow rate is detected by the detector 9, and the outlet temperature is detected by the detector 10. And, signals from each detector are put, together with the signal of the detector 7 to detect the temperature of a compound, in the arithmetic unit 15 as the measured signals 11W14 to obtain the load of the heat exchanger by means of a given formula. Then, using the deviation of load obtained for the heat exchanger, the controlled amount, e.g., speed, shape, the amount of heating medium, and flow rate, is obtained by a given formula and then sent to the analog regulator 17 as the output damand signal 16. Subsequently, by the signal, the control signal 18 is sent out to actuate the heating medium addition and flow rate controlling valve 19.