A heat pump device equipped with a heat storage tank for storing a heat-storing material having a thermosensitive polymer gel which contains a thermosensitive polymer which exhibits hydrophilic and hydrophobic properties depending on the temperature thereof, and also contains a solvent selected from the group consisting of water, an organic solvent and a mixture thereof, and further equipped with a heat-storing heat source device for heating the heat-storing material, a water tank for storing water, and a passage pipe for connecting the heat storage tank and the water tank to one another, wherein the heat-storing material reversibly changes between hydrophilic and hydrophobic properties at the boundary of the lower-limit critical solution temperature thereof, and during the process of said change, the organic solvent contained in the thermosensitive polymer gel maintains a liquid state, and the water and the thermosensitive polymer which has changed to become hydrophobic separate from one another.