A device for monitoring the sealing state of a casing (2) of a transmission box (1) of a rotorcraft. An open airflow circuit (7) fitted with a suction generator member (10) is connected to a vent (5) of the casing (2). Regulator means (12) maintain stable suction in the airflow circuit (7) relative to a setpoint threshold (S), the regulator means being controlled in response to a pressure gauge (13) for measuring the suction generated inside the airflow circuit (7). The device is used on the ground to detect a source of leakage in the casing (2) and in flight to retain lubricant inside the casing (2) in the event of there being a source of leakage.