A groundwater thermal storage system (1;1a;1b;1c;1d) comprising: a first thermal storage tank that is configured to store groundwater as a hot heat source; a second thermal storage tank that is connected to the first thermal storage tank through circulating piping, and that is configured to store cold drainage of the groundwater as the hot heat source that is cooled by being used in heat exchange, as a cold heat source; a heat exchange means that is provided in the circulating piping, and that is configured to perform heat exchange between the cold heat source from the second thermal storage tank and a first heating medium to cool the first heating medium; and a first cooling means (20;21) configured to maintain a temperature of the first heating medium after cooling within a predetermined temperature range, wherein the system is configured such that hot drainage heated by heat exchange performed by the heat exchange means is stored in the first thermal storage tank as a hot heat source through the circulating piping, characterized by further comprising a water heat source type heat pump apparatus that is configured to heat a second heating medium by using a hot heat source from the first thermal storage tank as a heat source; and a first heating means configured to maintain temperature of the second heating medium after heating within a predetermined temperature range, wherein the system is configured such that cold drainage of groundwater after being used in the water heat source type heat pump apparatus as a heat source is stored in the second thermal storage tank as a cold heat source, and wherein the first cooling means is equipped with ability of the first heating means, and is a heat pump apparatus that has a cooling function and a heating function, and that allows a heating medium flowing into the heat pump apparatus to be switchable for cooling and heating. a first thermal storage tank that is configured to store groundwater as a hot heat source; a second thermal storage tank that is connected to the first thermal storage tank through circulating piping, and that is configured to store cold drainage of the groundwater as the hot heat source that is cooled by being used in heat exchange, as a cold heat source; a heat exchange means that is provided in the circulating piping, and that is configured to perform heat exchange between the cold heat source from the second thermal storage tank and a first heating medium to cool the first heating medium; and a first cooling means (20;21) configured to maintain a temperature of the first heating medium after cooling within a predetermined temperature range, wherein the system is configured such that hot drainage heated by heat exchange performed by the heat exchange means is stored in the first thermal storage tank as a hot heat source through the circulating piping, characterized by further comprising a water heat source type heat pump apparatus that is configured to heat a second heating medium by using a hot heat source from the first thermal storage tank as a heat source; and a first heating means configured to maintain temperature of the second heating medium after heating within a predetermined temperature range, wherein the system is configured such that cold drainage of groundwater after being used in the water heat source type heat pump apparatus as a heat source is stored in the second thermal storage tank as a cold heat source, and wherein the first cooling means is equipped with ability of the first heating means, and is a heat pump apparatus that has a cooling function and a heating function, and that allows a heating medium flowing into the heat pump apparatus to be switchable for cooling and heating.