PURPOSE: To stably control the temp. of a thermostatic device by setting the integrated manipulated variable of the PI or PID operation to a specified initial value along with the increase and decrease in the cooling capacity.
CONSTITUTION: The inside of the thermostatic device is kept at a target temp. by driving the heating means 13 and cooling means 15 for respectively heating and cooling the inside of the thermostatic device. An arithmetic unit 3 for performing PI or PID operation based on the temp. in the thermostatic device and the target temp. is provided in a control means 2, the heating means 13 is driven by the obtained manipulated variable, and the capacity of the cooling means 15 is increased or decreased, if necessary,. In addition, the means 5 and 6 for setting the respective initial integrated values to the arithmetic unit 3 when the capacity of the cooling means 15 is increased or decreased is furnished to the control means 2. As a result, the undershoot and overshoot due to excessive cooling or heating can be reduced, and the temp. of the thermostatic device can be stably controlled.