The present invention relates to an air conditioner using hot water heated by a solar heating system, the air conditioner comprising: a heater 10 that has a vacuous interior and a plurality of hot water branch pipes 13 immersed in the refrigerant; a condenser 30 connected with the heater 10 through a steam pipe L1; an ejector 20 mounted to the steam pipe L1; a flow regulator 40 installed on the exit side of the condenser 30; a refrigerant return line L2 connected to the flow regulator 40 to return the refrigerant to the heater 10; an evaporator 50 connected to the flow regulator 40 to receive condensate; a first steam supply line L4 connected with the evaporator 50 to supply the evaporated refrigerant to the ejector 20; and a second steam supply line L5 that is branched from the first steam supply line L4 and then connected with the steam pipe L1 and has a vacuum pump P2 installed therein. With the above configuration, the present invention can save energy by performing air conditioning using solar energy that is available in plenty during the summer, and continue air conditioning by maintaining the refrigeration cycle with the help of a built-in vacuum pump even when there is not enough solar energy.