An air conditioning and ventilating system comprising: an air conditioning device including a compressor and a heat exchanger configured to generate conditioned air by heat exchange with a refrigerant, and configured to send the conditioned air to an air conditioned space; a ventilation device configured to ventilate the air conditioned space; a refrigerant sensor configured to detect concentration of the refrigerant in the air conditioned space; and a control unit configured to control operations of the air conditioning device and the ventilation device, wherein on determination that the refrigerant concentration acquired from the refrigerant sensor exceeds a first predetermined value, the control unit sets an operation of the compressor of the air conditioning device to a stop state and sets the ventilation device to an operating state, on determination that the refrigerant concentration that has exceeded the first predetermined value becomes equal to or less than the first predetermined value, the control unit continues the stop state of the compressor of the air conditioning device and the operating state of the ventilation device until predetermined timing, characterized in that the predetermined timing is time when predetermined time elapses after the refrigerant concentration that has exceeded the first predetermined value becomes equal to or less than the first predetermined value, and the predetermined time is calculated based on at least one of volume of the air conditioned space, ventilation capacity of the ventilation device, refrigerant volume expected to leak to the air conditioned space, and refrigerant leakage velocity. an air conditioning device including a compressor and a heat exchanger configured to generate conditioned air by heat exchange with a refrigerant, and configured to send the conditioned air to an air conditioned space; a ventilation device configured to ventilate the air conditioned space; a refrigerant sensor configured to detect concentration of the refrigerant in the air conditioned space; and a control unit configured to control operations of the air conditioning device and the ventilation device, wherein on determination that the refrigerant concentration acquired from the refrigerant sensor exceeds a first predetermined value, the control unit sets an operation of the compressor of the air conditioning device to a stop state and sets the ventilation device to an operating state, on determination that the refrigerant concentration that has exceeded the first predetermined value becomes equal to or less than the first predetermined value, the control unit continues the stop state of the compressor of the air conditioning device and the operating state of the ventilation device until predetermined timing, characterized in that the predetermined timing is time when predetermined time elapses after the refrigerant concentration that has exceeded the first predetermined value becomes equal to or less than the first predetermined value, and the predetermined time is calculated based on at least one of volume of the air conditioned space, ventilation capacity of the ventilation device, refrigerant volume expected to leak to the air conditioned space, and refrigerant leakage velocity.