To allow designing of a soil heat source heat pump system with consideration given to flow of underground water and to calculate underground temperature with practical precision and at a practical speed when underground water flows.

The performance prediction system for predicting performance of the soil heat source heat pump system, which uses a heat pump to circulate a heat medium in an underground heat exchanger by using the ground as a heat source for collecting/dissipating heat and supplies heat or cold to a load side, is provided with an analysis part 31 simulating operation of the soil heat source heat pump system to calculate underground temperature. In calculation of the underground temperature by the analysis part 31, a temperature response in the presence of underground water flow is found by multiplying a temperature response, which is given when no underwater flow based on the infinite cylinder surface heat flow response theory, by a previously obtained correction factor.