PURPOSE: To obtain a temperature control wherein a fluid is stabilized by a method wherein changes in water temperature of a fluid are sensed so that a hunting state is rapidly sensed to change over to a control method wherein a operation constant of PI control is changed.

CONSTITUTION: The flow rate F on the side of a fluid of a heat exchanger 1 is controlled by a three-way valve 2 to control fluid temperature Twn. A control circuit 4 connects a temperature sensor 7a for sensing the temperature Twn and a temperature sensor 7b for sensing temperature Twc of cooling water to compute a valve opening required corresponding to the deviation of the temperature Twn from a set fluid temperature Twset and temperature differences between the temperature Twn and the temperature Twc to output the computation to a motor 3. At this moment, when a sensing exceeding an allowable width with respect to the temperature Twset has taken place and it is judged that a hunting has occurred, a changeover is made to a PI control of a hunting part wherein a control constant is changed. As a result, even if a hunting has occurred, a stable fluid temperature control which has been automatically restored can be continued.