Provided is a multi-split type air conditioning system AC in which a plurality of indoor units 30 are connected in parallel, and which comprises a receiver 20 installed upstream of an outdoor heat exchanger 12 that serves as an evaporator in heating operation; and an outdoor unit electronic expansion valves 21 installed between the receiver 20 and the outdoor heat exchanger 12, wherein a control unit 50 is provided with an in-heating-operation opening degree control mode for controlling the outdoor unit electronic expansion valve 21, the in-heating-operation opening degree control mode comprising an open-loop control that performs an operation for a predetermined time period with an outdoor unit electronic expansion valve interim opening degree set corresponding to the actual number of rotation of a variable-power operated type compressor 11 determined by the number of the indoor units 30 being in operation and outdoor air condition; and an outdoor air temperature, a total heat exchange capacity value of the indoor heat exchangers 31, a number of indoor heat exchangers 31 that are stopped, suction superheating and discharge superheating of the variable-power operated type compressor 11 are employed as parameters for calculating the outdoor unit electronic expansion valve interim opening degree.