An air conditioning system, comprising: a refrigeration cycle including a refrigerant circuit on an indoor unit side and an outdoor unit side connected via a refrigerant liquid line (4, 4A, 4B) and a refrigerant gas line (5, 5A, 5B); a plurality of outdoor units (2A, 2B) each including at least one compressor, a four-way selector valve configured to operate the refrigeration cycle to switch to any of a cooling cycle and a heating cycle, and an outdoor heat exchanger, and connected in parallel in the refrigeration cycle; and a backup operation control unit (61A, 61B) configured to perform backup operation on another operational outdoor unit to continue air conditioning operation when the compressor of any of the plurality of outdoor units (2A, 2B) becomes non-operational; characterized in that the backup operation control unit (61A, 61B) includes a four-way selector valve synchronization control unit (62A, 62B) configured to operate the four-way selector valve in backup operation to switch from a cooling cycle position to a heating cycle position, such that the four-way selector valve synchronization control unit (62A, 62B) operates the four-way selector valve of the operational outdoor unit temporarily in a cooling cycle position so as to reduce a low pressure of the non-operational outdoor unit and to secure an operating pressure difference of the four-way selector valve of the non-operational outdoor unit, and then operates the four-way selector valve of the non-operational outdoor unit to switch to a heating cycle position, and subsequently operates the four-way selector valve of the operational outdoor unit to switch to a heating cycle position and synchronizes the four-way selector valves so as to operate in a heating cycle. a refrigeration cycle including a refrigerant circuit on an indoor unit side and an outdoor unit side connected via a refrigerant liquid line (4, 4A, 4B) and a refrigerant gas line (5, 5A, 5B); a plurality of outdoor units (2A, 2B) each including at least one compressor, a four-way selector valve configured to operate the refrigeration cycle to switch to any of a cooling cycle and a heating cycle, and an outdoor heat exchanger, and connected in parallel in the refrigeration cycle; and a backup operation control unit (61A, 61B) configured to perform backup operation on another operational outdoor unit to continue air conditioning operation when the compressor of any of the plurality of outdoor units (2A, 2B) becomes non-operational; characterized in that the backup operation control unit (61A, 61B) includes a four-way selector valve synchronization control unit (62A, 62B) configured to operate the four-way selector valve in backup operation to switch from a cooling cycle position to a heating cycle position, such that the four-way selector valve synchronization control unit (62A, 62B) operates the four-way selector valve of the operational outdoor unit temporarily in a cooling cycle position so as to reduce a low pressure of the non-operational outdoor unit and to secure an operating pressure difference of the four-way selector valve of the non-operational outdoor unit, and then operates the four-way selector valve of the non-operational outdoor unit to switch to a heating cycle position, and subsequently operates the four-way selector valve of the operational outdoor unit to switch to a heating cycle position and synchronizes the four-way selector valves so as to operate in a heating cycle.