To solve a problem that a temperature difference of heat to be collected is small since a heat pump may be frozen when the groundwater of a heat pump system utilizing the groundwater is used as a heat source of the heat pump, and further the total length of an underground heat exchanger is increased since the quantity of heat absorbed from the ground is small when the brine is used as heat source water in a heat pump system utilizing ground heat, thus causing a cost increase.

The brine is used as the heat source water of a water heat source heat pump. The brine absorbs heat from the groundwater and the ground in a returning well-groundwater well heat exchanger, and further exchanges heat with the groundwater in a water-water heat exchanger to absorb heat from the groundwater. Further a returning well-groundwater well device constituted by inserting an U-tube heat exchanger into the returning well for returning the groundwater to an original groundwater vein of the groundwater extracted from the groundwater vein, charging a water permeable filler material between the outside of the U-tube and a casing, and holding a cover for preventing the overflow of water from a ground surface is used.