To achieve further power saving by not only stopping part of an internal circuit in a communication device but also stopping a communication unit in the communication device.

In a communication system in which a central device and communication terminals, connected with each other in a multi-drop manner through a communication cable, each of which has a normal mode and a power saving mode, each terminal device includes: first control means for performing control of communication with the central device via the cable; second control means responsible for control the first control means does not perform; clock supply means for supplying an operation clock to the first control means; and an interruption switch for switching from the power saving mode to the normal mode. The first control means comprises: receiving means for receiving a power saving mode switch signal from the central device; transmitting means for transmitting a mode switching signal to the central device; state transition means for instructing the transmitting means to transmit a signal indicating execution of mode switch, and also for instructing the clock supply means to execute mode switch; and detection means for detecting whether or not a voltage change of the communication cable exceeds a predetermined threshold value.