The invention relates to a directional coupling-type multi-drop bus of which the impedance is matched with the bus at the time of coupling so that the speed is increased. A directional coupler is formed when a second module provided with a second coupler end is mounted on a first module provided with a first coupler end, and as a result, the coupling impedance where the proximity effects in the coupling state of the directional coupler are reflected is matched with the impedance of the bus.