To reduce consumption of power from a commercial power source by a comparatively simple system utilizing inexpensive ground heat.

This ground heat utilization heat pump unit system includes an in-ground heat exchange pipe 141, a heat pump unit 110, tank circulation circuits 145, 146 for delivering a heat medium in a heat medium tank 150 to the heat pump unit, a warm water tank 170, a warm water tank circuit 171 for circulating the heat medium from the heat pump unit to the warm water tank, and heating/cooling circulation circuits 161, 162 for taking out the heat medium from the heat medium tank, and delivering the same to a heat absorption/radiation panel, and the heat pump unit heats the heat medium by delivering the heat medium at a lower part of the heat medium tank to the heat pump unit, when the in-ground heat exchange pipe is connected to a heat radiation circuit pipe 121 of the heat pump unit, and a heat absorption circuit pipe 125 for heating the heat medium to the tank circulation circuit, and lowers a temperature of the heat medium by delivering the heat medium at an upper part of the heat medium tank to the heat pump unit, when the in-ground heat exchange pipe is connected to the heat absorption circuit pipe, and the heat radiation circuit pipe is connected to the tank circulation circuit.