Abstract: A drainage system installed in a building for conducting waste water from such sanitary fixtures as toilet bowls into the underground sewer, has a stack extending through the floor of each story of the building and branch pipes for conducting the waste water from the fixtures mounted on each floor into the stack through connectors. The stack and each connector have a double structure comprising inner and outer members. The inside of the inner member is used for the discharge of waste water, while the space formed between the inner and the outer member communicates with the inside of the inner member within each connector to permit the flow of air necessary for eliminating a pressure variation induced inside the inner members by the flow of waste water.