Conventional geothermal energy development has targeted either the geothermal fluid reservoir in which steam and hot water are dominant or a deep-seated hot dry rock. These geothermal resources are classified into deep-seated geothermal ones and are restricted regionally. On the other hand, even though shallow geothermal resources are low-grade energy, they are not only exploited with comparative ease but also can be found anywhere. For this reason extensive studies on the effective use of shallow geothermal resources have been carried out in conjunction with the ground source heat pump system for several decades. In the ground heat pump system, the U-tube has been widely used in general, as a downhole heat exchanger because of its reasonable construction cost and easy construction. Furthermore, from the viewpoint of heat exchange area, a double U-tube downhole heat exchanger has a thermal advantage over a single U-tube downhole heat exchanger. The objective of the present study is to investigate the heat extraction characteristics of a double U-tube downhole heat exchanger with a numerical simulation. Firstly, a computer simulation program for predicting the heat extraction performance of a double U-tube downhole heat exchanger was produced. Secondly, the present computer simulation program was tested, and the predicted result was compared to the experimental ones obtained by other researchers to verify the effectiveness of the present computer simulation program. Thirdly, a numerical simulation has been conducted for the real scale model of a double U-tube downhole heat exchanger with the present program under a variety of numerical conditions. From the computed results it was found that increasing both the flow rate of the working fluid of heat extraction and the thermal conductivity of the U-tube increases the heat extraction rate under the prescribed numerical conditions in the present study. The comparison of heat extraction performance between a single and double U-tube downhole heat exchangers was made, and it revealed that the heat extraction rate using a double U-tube downhole heat exchanger is greater than that using a single U-tube under a fixed flow rate of the working fluid of heat extraction. The effect of the U-tube arrangement on the eat extraction rate was also shown in this study.