PURPOSE: To enable to tolerate the drastic change of set value and the lag of the timing of change of set value by a method wherein the mutual relativity is established between a primary and a secondary controlling systems by integrating the ratio of primary side flow rate signal to secondary side flow rate signal into the controlling system.

CONSTITUTION: Respective proportional-plus-integral-plus-derivative controllers 9 and 16, which respectively operate a primary pump 2 in a primary loop 4 and a secondary pump 3 in a secondary loop 11 by means of the inputs of the respective deviation signals 8 and 15 between the detection signal 6 of a flow rate detecting means 5 equipped within the primary loop 4 and between the detection signal 13 of a flow rate detecting means 12 equipped within the secondary loop 11 of the heat exchanger 1, are provided in the controlling device of the heat exchanger 1. In said controlling device of the heat exchanger 1, the ratio of flow rates detected with the means 5 and 12 is calculated at an arithmetic unit 18 and as well as the flow rate ratio calculated at the unit 18 is applied to function generator 20 in order to calculate parameters of controlling system. After that, the inputs to the controllers 9 and 16 are respectively corrected at respective deviation correcting units 22 and 23 based upon the output of the function generator 20.