To provide a water-refrigerant heat exchanger demonstrating good heat exchange performance by using plate heat exchanger as a condenser of the water refrigerant heat exchanger.

In a water-refrigerant heat exchanger, a plurality of approximately flat heat transfer plates are superposed with spaces so as to form refrigerant passages and water passages alternately between the heat transfer plates and refrigerant inlet/outlet openings and a water inlet/outlet openings are provided. A unit flow passage that functions as a passage for a heat source fluid such as water or the like is formed outside the heat transfer plates by the refrigerant passages sandwiched between the two heat transfer plates, and a plurality of the units flow passages are assembled to form assembled refrigerant passages 35, 36, 37, 38. Flow directions of these assembled flow passages 35, 36, 37, 38 are reversed alternately with each other so as to communicate in series, and the number of the unit flow passages of the assembled refrigerant flow passages 35, 36, 37, 38 are decreased as 4, 2, 1, 1 from one refrigerant opening to the other refrigerant opening.