Water heater of heat pump

The invention provides a water heater of a heat pump, wherein the scale is effectively prevented from precipitating and adhering at the high temperature part of a heat-conduction pipe at the high-temperature side. The water heater of the heat pump comprises a compressor, a second water-refrigerant heat exchanger, a first water-refrigerant heat exchanger, a pressure reducing device, a heat pump refrigerant loop connected with an air heat exchanger in series and a water heating loop. The first water-refrigerant heat exchanger is connected with the second water-refrigerant heat exchanger in series by the water heating loop. Water is heated by the water heating loop. The water heater is characterized in that the inner surface of the heat-conduction pipe of the second water-refrigerant heat exchanger is equipped with concavo-convex. Or the water heater is characterized in that the inner side of the heat-conduction pipe of the second water-refrigerant heat exchanger is equipped with a movable member and the movable member is operated by the flowing of water.