PURPOSE: To enable to vent air in a short time by a method wherein the liquid is sucked from a vessel by an air venting pump when a valve connected to a branched joint is ON, in the titled system useful for a liquid chromatograph.

CONSTITUTION: Developing solution is entered into the air vent pump 14 by a developing solution feeding pump 3 from the vessel C1 through a three-way branched joint 10, a drain valve 11, a tube 15 and an inflow pipe to discharge the particles of air adhered to the wall of the tube 15 together with the developing solution. When the drain valve 11 is closed to analyze the specimen, simultaneously, another drain valve 13 is opened and the pump 14 is operated, the particles of the air adhered to the wall of the tube between an air trap T2 and the pump 6, the wall of the cylinder of the pump 6 and the wall of the tube between the pump and the joint 12 may be discharged.