This paper discusses a developed simulation tool for ground-source heat pump (GSHP) systems with multiple ground heat exchangers (GHEXs) by combining the infinite cylindrical source (ICS) solution with the infinite line source (ILS) solution. In the developed tool, superposition of the temperature field in space was applied to calculate the underground temperature due to heat injection from multiple GHEXs. Then, the ICS solution was applied for temperature change due to heat injection of the considered GHEX and the ILS solution was applied for temperature change due to the heat injection of other neighboring GHEXs. In this paper, first it is shown that the temperature response of the ICS can be regarded as that of the ILS in cases where the distance from the heat source to the considered point becomes larger. Next, the method for the high-speed calculation of temperature change due to heat injection of the multiple GHEXs is described. In addition, the advantages of the calcu- lation method in the developed tool are introduced. Finally, considering the case of a GSHP system installed in an office building in Tokyo, the temperature change of the primary side of the heat pump is calculated, and its reproducibility is verified by comparison with measured values. The results show the difference between the calculated and measured values to be up to 5°C (9°F) and less than 2°C (3.6°F) in most cases, thus confirming the reproducibility of this tool.