To transmit a digital signal by way of a simple circuit means and a few signal lines.

A transmission unit 1 accumulates digital transmission signals S11S1n so that they are a multi-level signal (m) with plurality of logic signal levels. The multi-level signal (m) is sent from the transmission unit 1 to a reception unit 3 via a signal line 2 and compared with signal threshold values V1...Vn between the logic signal levels of the multi-level signal (m) to form digital reception signals S31v3n equivalent to the digital transmission signals S11...S1n.