The pouring spout has an inlet tube detachably connectable to a liquid-storage container and an outlet tube slideably connected to the inlet tube. A liquid duct and an air duct are defined into the inlet tube and the outlet tube. A valve system is provided for opening the liquid duct and the air duct when the outlet tube is slideably moved relative to the inlet tube. The liquid duct and the air duct open separately into the liquid-storage container. When pouring, the liquid flows through the liquid duct and air separately enters into the liquid-storage container via the air duct.