A communication system includes one or more communication apparatuses, each including a transmission circuit and a reception circuit. The transmission circuit includes a transmission unit and a signal generator. The transmission unit transmits one of a first transmission signal including first transmission data or a second transmission signal including at least one of a periodic signal with a constant frequency or a narrow band modulation signal having lower speed than the first transmission data. The signal generator generates the second transmission signal. The reception circuit includes a reception unit that receives the first transmission signal and the second transmission signal a narrow band detector that outputs a narrow band detection signal through detecting the second transmission signal in a narrow band, and a determination unit that determines presence of a significant signal or presence of significant signal change in the narrow band detection signal and outputs a determination signal.