It is intended to realize an air conditioning system of high operational efficiency without reducing the user's comfort. In an air conditioning system 1, a set temperature determining device 31 determines a target temperature of water to be supplied to an indoor heat exchanger 12, based on [“target outflow temperature”=“current outflow temperature”+((“inlet and outlet temperature difference”/“indoor and outdoor temperature difference”)דset temperature difference”)]. The indoor and outdoor temperature difference is a difference between an indoor temperature and an outdoor temperature, the inlet and outlet temperature difference is a difference between temperatures of water at the inlet side and the outlet side of an intermediate heat exchanger 9, and the set temperature difference is a difference between an indoor temperature and a set temperature. A control device 32 controls an outdoor unit 2 in response to the target temperature determined by the set temperature determining device 31.