Switch units control ON and OFF of communication between a master device and slave devices. Signal levels of communication lines between switch units and slave devices are maintained at a predetermined level. A switch unit in a first stage is turned on and off by a first switch control signal. A switch unit in a second stage is turned on and off by a switch control signal generated based on a level of a communication line in the first stage and the first switch control signal. A switch unit in each of subsequent stages is turned on and off by a switch control signal generated based on a level of a communication line in a preceding stage and a switch control signal in a second preceding stage. A portion of ground short circuit is identified based on states of communication between the master device and the slave devices.