PURPOSE: To enable to fit a solar heat collector at a position as high as a pump with a smaller life is possible by a strucure wherein a by-pass passage, the passage resistance of which is larger than that of an heat exchanger, is provided in parallel to the heat exchanger.

CONSTITUTION: A by-pass passage 20, the passage resistance of which is larger than that of an heat exchanger 4, is provided in parallel to the heat exchanger 4. Due to the structure as mentioned above, once when the filling of water in a system is performed, water is automatically filled in the system, even if the falling of water occurs afterwards, and the collection of heat can be performed with no trouble. Accordingly, a solar heat collector 11 can be fitted at a higher position by employing a pump 6 with a smaller lift.