Abstract: A pressure control system for stern tube seals includes at least two resilient radial lip-type seal rings held so as to surround a propeller shaft linear. The closest seal ring lip to the propeller is arranged to face outboard and the other adjacent seal rings' lips are arranged to face suitable directions so as to form annular chambers. The aft annular chamber located most closely to the propeller serves as a chamber having a fixed differencial pressure higher than the seawater pressure. The closest seal ring to the propeller is lifted, thereby allowing air to be bled into seawater and preventing the ingress thereof. A regulator unit which is capable of instantly ascertaining changes in the hull's draft is provided in an air-supplying mechanism communicating with the aft annular chamber, so as to maintain the air pressure therein higher than the seawater pressure by a constant differential pressure.