A communication system includes: a first signal line; a second signal line; a first signal line selection portion and a second signal line selection portion for selecting either one of the first signal line and the second signal line so as to be effective as the serial data; and a signal control portion for outputting a signal direction switching signal for controlling a selecting operation. The first signal line selection portion includes a first OR circuit and a first three-state buffer circuit provided on the first signal line, and a first buffer provided on the second signal line, and the second signal line selection portion includes a second OR circuit and a second three-state buffer circuit provided on the second signal line, a second buffer provided on the first signal line, and an inverter circuit provided between a terminal of the signal control portion and the second OR circuit.