The specification of the underground heat exchanger used for the ground source heat pump that uses direct expansion method could not be established so far since the refrigerant flowing in the underground heat exchanger is a gas-liquid two-phase flow accompanying evaporation and condensation and its behavior is also complicated. Therefore, an experiment was conducted using a U-shaped heat exchanger in which one 3/8-inch copper tube and multiple 1/4-inch copper tubes were connected at the bottom as a heat-collecting tube. In this experiment, the parallel underground heat exchanger were used. We introduced this underground heat exchanger into the air conditioner used for actual agricultural vinyl house, and describe the results of the demonstration test. The COP in the cooling operation was about 4.0 and in the heating operation it was about 3.5, and the primary energy consumption reduction rate compared with the conventional system was 20% and 43% respectively.