An air conditioning system (1; 100) comprising: a refrigeration cycle to circulate a slightly flammable refrigerant or a flammable refrigerant, and an indoor unit including a heat exchanger connected to the refrigeration cycle, the indoor unit being installed in an indoor space, the air conditioning system (1; 100) comprising: a refrigerant sensor that detects the refrigerant leaking from the indoor unit, a ventilation device that ventilates the indoor space, a power storage unit that is charged by a commercial power source, and a controller, wherein the refrigerant sensor and the ventilation device are operated by the commercial power source, characterized in that the controller operates the ventilation device with power of the power storage unit, regardless of whether or not the refrigerant leaking is detected by the refrigerant sensor, in a case where the controller detects that a power supply from the commercial power source is stopped; the controller operates the refrigerant sensor with power of the power storage unit and determines whether or not the refrigerant sensor normally functions when the power supply from the commercial power source does not recover from the power failure after operating the ventilation device; and the controller executes a first ventilation operation to operate the ventilation device with power of the power storage unit for a predetermined ventilation time that is calculated based on a remaining amount of power in the power storage unit, a volume in the indoor space, and a ventilation amount of the ventilation device per unit time in a case where the controller determines that the refrigerant sensor normally functions. a refrigeration cycle to circulate a slightly flammable refrigerant or a flammable refrigerant, and an indoor unit including a heat exchanger connected to the refrigeration cycle, the indoor unit being installed in an indoor space, the air conditioning system (1; 100) comprising: a refrigerant sensor that detects the refrigerant leaking from the indoor unit, a ventilation device that ventilates the indoor space, a power storage unit that is charged by a commercial power source, and a controller, wherein the refrigerant sensor and the ventilation device are operated by the commercial power source, characterized in that the controller operates the ventilation device with power of the power storage unit, regardless of whether or not the refrigerant leaking is detected by the refrigerant sensor, in a case where the controller detects that a power supply from the commercial power source is stopped; the controller operates the refrigerant sensor with power of the power storage unit and determines whether or not the refrigerant sensor normally functions when the power supply from the commercial power source does not recover from the power failure after operating the ventilation device; and the controller executes a first ventilation operation to operate the ventilation device with power of the power storage unit for a predetermined ventilation time that is calculated based on a remaining amount of power in the power storage unit, a volume in the indoor space, and a ventilation amount of the ventilation device per unit time in a case where the controller determines that the refrigerant sensor normally functions. a refrigerant sensor that detects the refrigerant leaking from the indoor unit, a ventilation device that ventilates the indoor space, a power storage unit that is charged by a commercial power source, and a controller, wherein the refrigerant sensor and the ventilation device are operated by the commercial power source, characterized in that the controller operates the ventilation device with power of the power storage unit, regardless of whether or not the refrigerant leaking is detected by the refrigerant sensor, in a case where the controller detects that a power supply from the commercial power source is stopped; the controller operates the refrigerant sensor with power of the power storage unit and determines whether or not the refrigerant sensor normally functions when the power supply from the commercial power source does not recover from the power failure after operating the ventilation device; and the controller executes a first ventilation operation to operate the ventilation device with power of the power storage unit for a predetermined ventilation time that is calculated based on a remaining amount of power in the power storage unit, a volume in the indoor space, and a ventilation amount of the ventilation device per unit time in a case where the controller determines that the refrigerant sensor normally functions.