An air conditioning system equipped with at least one first air conditioning system in which an outdoor unit is connected to plural indoor units through an inter-unit pipe and the plural indoor units are allowed to execute one of cooling operation and heating operation concurrently, at least one second air conditioning system in which an outdoor unit is connected to plural indoor units through an inter-unit pipe and the plural indoor units are allowed to execute both cooling operation and heating operation in mixture concurrently, and a controller that is connected to the first and second air conditioning systems through a communication line so that communications can be performed between the controller and each of the first and second air conditioning systems, wherein the controller transmits a switching instruction signal for instructing switching of an air conditioning operation to cooling operation or heating operation through the communication line to the first and second air conditioning systems all together.