Abstract: A sewer line backup detection device and alarm are provided which can be installed in place of the cleanout access cap in a conventional sewer line. The device comprises a housing having male connecting threads adapted to fit standard female receptacles which have been used in the past to provide cleanout access to sewer lines, the housing containing alarm means or alarm transmitting means actuated by a pneumatic switch. The pneumatic switch is actuated by a flexible diaphragm which is fitted snugly onto the lower end of the housing which extends into the sewer line. In the event of a blockage of the sewer line, the blocked material exerts pressure on the flexible diaphragm which closes the pneumatic switch thereby setting off an alarm, either locally or at a remote location such as the dwelling from which the sewer line exhausts. The alarm system is utilized especially advantageously in combination with a detention tank located in the basement or lower level of a high rise building.