PURPOSE: To simplify the mechanism of a transmission line between moving bodies by converting the states of plural switches, provided to one moving body, into serial codes, and then transmitting them to the other moving body.

CONSTITUTION: Plural switches S1∼Sn such as horn switch are provided at the part of a steering wheel. The states of the respective switches are converted by the encoder 2 composed of a Johnson counter into serial codes, which are outputted. This serial signal is amplitude-modulated by a modulator, whose output is transmitted to a transmission line through a capacitor. The transmission line is connected to the outside through a slip ring. The transmitter transmits a direct current for power supply simultaneously with the signal. Thus, plural signals are transmitted through one slip ring.