To provide an absorption air conditioning system and cooling/heating change-over method which do not make users uncomfortable just after a cooling mode is changed to a heating mode and vice versa, there is provided an absorption air conditioning system including an absorption cool water/hot water making machine for producing cool water/hot water for air conditioning, a room machine for carrying out the air conditioning of a room by circulation of the cool water/hot water from the absorption cool water/hot water making machine, and a piping for introducing the cool water/hot water from the absorption cool water/hot water making machine to the room machine, wherein the system further includes a bypass piping connected between the absorption cool water/hot water making machine and the room machine through a valve, and a means for opening/closing the valve.