Air-condition systems with Ground Source Heat Pumps (GSHP) system is known to be energy efficient and is well distributed in American and European countries. On the other hand, the number of the systems is still limited in Japan due to the lack of the knowledge of the optimum design for the system. For the further promotion of the GSHP system, the high efficiency of the GSHP system for air-conditioning needs to be proved by improving the performance of heat exchange. The objective of this study is to evaluate the performance of GSHP system installed in a large public facility in Akita, Japan. This facility has 36 boreholes with single U-tubes for heating and cooling purposes. The area for air condition in this facility is around 1530 m2. Thermal Response Test (TRT) showed that the ground thermal conductivity in this site is 1.70 W/m/K. The geological column shows the existence of a gravel layer at around 80 m. The inlet temperatures, the outlet temperatures and the flow rate during operation of GSHP system have been recorded since April, 2015. Based on the analysis of operation behavior for 1 year, the cumulative heat extracted and heat injected were calculated as 93.95 MWh and 25.68 MWh, respectively, indicating the poor balance in the heat exchange rates in summer and winter. A numerical model of the single ground heat exchanger was then developed using FEFLOW ver.7.0 software. The validity of this model was confirmed by conducting history matching with TRT data. Next, a numerical model of the borehole field was developed based on the parameters of the single well model. To confirm the validity of borehole filed model, history matching with operation data was conducted with reasonably good agreement. The long-term simulations with the borehole field model showed that the operation behavior would be stable for 10 years under current operation condition and the ground temperature around Boreholes Heat Exchangers (BHEs) would be affected by the direction of groundwater flow and the position of BHEs.