Abstract: The leakage check system is applied to a vessel having a sealing valve unit that can close a gas passage and a liquid passage of the outlet port of the vessel. To determine the presence of a liquid leakage from the outlet port, there is provided an electrode for measuring liquid electrical conductivity. An actuator moves the electrode forward and backward relative to the outlet port. A forward part of the electrode is surrounded by a liquid receiver cup which is opposed to the outlet port during the leakage check of the liquid in the vessel. The forward part of the electrode is extended from a bottom of the liquid receiving cup. The liquid receiving cup is supplied with a dilution water from a water feeder. The vessel is conveyed by a conveyor with the vessel being upside down so that the outlet port is directed downwardly. The outlet port of the vessel is insertable into the liquid receiving cup.