A communication apparatus (100) of the present invention includes a first communication circuit (2), a second communication circuit (1), and one signal line (3). The first communication circuit (2) transmits a collector voltage of an open collector circuit as an output signal. A second communication circuit (1) receives the output signal. The one signal line (3) connects the first communication circuit (2) and the second communication circuit (1). Particularly the first communication circuit (2) transmits the output signal as a pulse signal (57) to the second communication circuit (1). The second communication circuit (1) transmits a voltage signal (56) generated in the second communication circuit (1) to the first communication circuit (2). The first communication circuit (2) and the second communication circuit (1) communicate the pulse signal (57) and the voltage signal (56) bidirectionally via the signal line (3).