To provide a geothermal heat pump device stably operable over a long time adaptedly to a thermal situation of the ground, and to provide its control method.

A limit value of heat collection and heat radiation is set based on the outlet temperature of heat medium water from a heat pump 3 to an underground heat exchanger 2 measured with a temperature sensor 14, and the operation of the heat pump 3 is controlled not to exceed the set limit value of the heat collection and heat radiation. Thus, the underground temperature can be prevented from becoming too high (or low) to extremely decrease the heat collection/radiation efficiency during the operation of an air conditioner, the underground temperature can be prevented from gradually increasing (or decreasing) over a long time and disabling the heat collection/radiation, and hence stable operation can be achieved over a long time adaptedly to the thermal situation of the ground 3.