An air-conditioning system, comprising: an air conditioner including a refrigerant circuit formed by connecting an outdoor unit and a plurality of indoor units together, the air conditioner being configured to condition air in a plurality of indoor spaces; at least one countermeasure device (V, 65) provided in correspondence with at least one of the plurality of indoor spaces and configured to operate during leakage of a refrigerant; a plurality of detectors configured to detect a concentration of the refrigerant, at least one of the detectors being provided in each of the plurality of indoor spaces; and a control unit configured to operate all of the at least one countermeasure device (V, 65) if at least one of the concentrations of the refrigerant respectively detected by the plurality of detectors exceeds a predetermined value, wherein the control unit includes a relay control unit, and a plurality of indoor controllers each provided in an associated one of the indoor units, at least one of the indoor controllers is capable of communicating with the at least one countermeasure device (V, 65), each indoor controller, if the concentration of the refrigerant detected by the detector provided in the indoor space where the indoor controller is located exceeds the predetermined value and the at least one countermeasure device (V, 65) capable of communicating with the indoor controller is present, operates the at least one countermeasure device (V, 65) capable of communicating with the indoor controller, and transmits a first signal to the relay control unit, and the refrigerant circuit includes indoor circuits each provided in an associated one of the indoor units, first refrigerant flow paths each connected to a liquid end portion of an associated one of the indoor circuits, and second refrigerant flow paths each connected to a gas end portion of an associated one of the indoor circuits, characterized in that the at least one countermeasure device (V, 65) includes a plurality of countermeasure devices (V, 65), and the relay control unit, when receiving the first signal, operates all of the countermeasure devices (V, 65) except the at least one countermeasure device (V, 65) corresponding to the indoor controller that has transmitted the first signal, and the at least one countermeasure device (V, 65) is at least one of a cut-off valve provided in one or each of the associated first and second refrigerant flow paths and, or a ventilator configured to ventilate an associated one of the indoor spaces. an air conditioner including a refrigerant circuit formed by connecting an outdoor unit and a plurality of indoor units together, the air conditioner being configured to condition air in a plurality of indoor spaces; at least one countermeasure device (V, 65) provided in correspondence with at least one of the plurality of indoor spaces and configured to operate during leakage of a refrigerant; a plurality of detectors configured to detect a concentration of the refrigerant, at least one of the detectors being provided in each of the plurality of indoor spaces; and a control unit configured to operate all of the at least one countermeasure device (V, 65) if at least one of the concentrations of the refrigerant respectively detected by the plurality of detectors exceeds a predetermined value, wherein the control unit includes a relay control unit, and a plurality of indoor controllers each provided in an associated one of the indoor units, at least one of the indoor controllers is capable of communicating with the at least one countermeasure device (V, 65), each indoor controller, if the concentration of the refrigerant detected by the detector provided in the indoor space where the indoor controller is located exceeds the predetermined value and the at least one countermeasure device (V, 65) capable of communicating with the indoor controller is present, operates the at least one countermeasure device (V, 65) capable of communicating with the indoor controller, and transmits a first signal to the relay control unit, and the refrigerant circuit includes indoor circuits each provided in an associated one of the indoor units, first refrigerant flow paths each connected to a liquid end portion of an associated one of the indoor circuits, and second refrigerant flow paths each connected to a gas end portion of an associated one of the indoor circuits, characterized in that the at least one countermeasure device (V, 65) includes a plurality of countermeasure devices (V, 65), and the relay control unit, when receiving the first signal, operates all of the countermeasure devices (V, 65) except the at least one countermeasure device (V, 65) corresponding to the indoor controller that has transmitted the first signal, and the at least one countermeasure device (V, 65) is at least one of a cut-off valve provided in one or each of the associated first and second refrigerant flow paths and, or a ventilator configured to ventilate an associated one of the indoor spaces.