The purpose of this study is to investigate a performance of the ground source heat pump that uses direct expansion method in case of hot water supply. The performance of cooling and heating / hot water supply systems of ground source heat pump that uses direct expansion method will be higher than that of conventional ground source heat pump. In consideration of actual use conditions, experiment was conducted by using R32. The length of the underground heat exchanger is 30 m. We investigated a maximum value of the amount of extracted heat from the ground by the single underground heat exchanger. The experiment was continued until the temperature of hot water in the tank reached about 65 °C. When the discharge amount of hot water was 200 L, the average COP during the experiment was 2.4. When the discharge amount was 300 L, the average COP was 2.3.