A communication system capable of communicating control information without decreasing the bandwidth available to users. A coder encodes a primary signal to generate encoded data such that the polarity of the encoded data alternates between positive and negative polarities. A polarity-reversed data generator reverses the polarity of part of the encoded data to generate polarity-reversed data. A transmitter transmits, as communication data, the encoded data including the polarity-reversed data. A receiver receives the communication data, and a polarity-reversed data detector determines whether or not the polarity of the received communication data alternates between positive and negative polarities, and detects, as the polarity-reversed data, data of which the polarity differs from an expected polarity. A polarity recognizer recognizes the polarity of the polarity-reversed data and treats the recognized polarity as control information.