To realize a CO2 heat pump system in which reduction in coefficient of performance of heat pump cycle is prevented by maintaining the temperature of the water supplied to the gas cooler of the CO2 heat pump below a certain temperature and raising heat source side temperature in the evaporator, and which has functions to supply a heat source not only to hot water supplying equipment but also to supply a heat source or coolness source to air conditioning equipment or others, the system comprises a low temperature water tank 6, a high temperature water tank 9 for storing high temperature water heated in a gas cooler 3, piping 10, 12 for supplying the high temperature water to hot water supplying equipment and room heating equipment respectively, a first heat exchanger 21 in which brine (water) for supplying latent heat of vaporization of CO2 refrigerant to the refrigerant in the evaporator 5 exchanges heat with heat giving fluid (fluid from which heat is withdrawn), a second heat exchanger located upstream of the first heat exchanger to cool the high temperature water after it is used as a heat source for the room heating equipment (therefore the temperature is already not so high) by heat exchange with the heat receiving fluid, piping for introducing the water cooled in the second heat exchanger to the low temperature water tank 6, and a low temperature water supply means 7.