A nipple with a developed pressure adjusting valve mounted in the bottom of air cavity of a mounting flange and shaped like a hollowed cylinder having a closed end, with the inside of the valve is used as a valve cavity for air flowing. A side wall of the valve above the bottom-closed end is formed with at least one slit at a part of periphery of the valve. Through the airflow passageway of the valve, the slit serves as an open-closed door to allow air to flow into the liquid-containing bottle advantageously, the air can flow into the liquid-containing bottle easily and conveniently, allowing the liquid to flow out from the bottle continuously and steadily, preventing the nipple from collapsing and preventing a vacuum formed inside the bottle as well as preventing the leakage of liquid from the valve when the bottle is inverted or sucked.
