The invention relates to a respiratory gas supply circuit for an aircraft carrying passengers and/or crewmembers (1) in a cabin, including a source of breathable gas (R1, R2), at least one supply line (20) connected to said source, a first actuating device (60, 12) provided on said supply line to open at least partially or close the supply of breathable gas, a respiratory device (50) adapted to be worn on one passenger or crewmember's face, said respiratory device being connected to said supply line, and further including an ambient air inlet (40), said respiratory device providing to said one passenger or crewmember a respiratory gas to be inhaled corresponding to ambient air and/or breathable gas, wherein said ambient air inlet further includes a second actuating device (4, 11) to open at least partially or close said ambient air inlet; and, both said first and second actuating devices are operated so that at least one of said first and second actuating devices is closed at each instant of time.