PURPOSE: To prevent a return water temperature from being lowered even in the wintertime by controlling a pressure regulating mechanism to reduce a water feed pressure in the case where the return water temperature is lower than a predetermined level.

CONSTITUTION: In the case where a temperature of return water in a return water passage 12 is lowered in accordance with a temperature decrease of the external air to be introduced into air conditioning means A1 to An during the wintertime and the like, a signal of the decrease of the return water temperature is inputted into a temperature regulator 11, and a pressure setting signal i.e., a pressure reducing signal is outputted to a pressure regulator 10 on the basis of the differences between the return water temperature decreased and a preset temperature. The pressure regulator 10 makes the opening degree of a bypass valve 9 larger to reduce a water feed pressure in a water feed passage 6. With this arrangement, even when a temperature of return water flowing through the return water passage 12 becomes lower than a predetermined level, the temperature of return water can be increased by the control of a pressure regulating mechanism.