PURPOSE: To turn on and off an electric power source of the whole communication equipment by a simple operation, by applying DC voltage for controlling the electric power source, to a transmission line when an electric power supply is in a turning-off state in one of a communication equipment or an additional apparatus, and on the other hand, detecting the applied voltage and turning off the electric power supply.

CONSTITUTION: Coupling capacitors 15, 25, electric power supplying circuits 16, 26, AND gates 17, 27, control voltage detecting circuits 18, 28, control voltage generating circuits 19, 29, etc. are provided on a radio device 1 and an additional apparatus 2, respectively, and both of them are connected by an input signal line 3. When one switch 14 or 24 of this communication equipment 1 or the additional apparatus 2 becomes off, the gates 17, 27 send out a stop signal to the circuits 16, 26 and stop the operation, and also the circuits 19, 29 apply DC voltage for controlling the electric power source to the signal line 3. This voltage is detected by the circuits 28, 18, is supplied to the gates 27, 17, voltage conversion and stabilizing operation of the circuits 26, 16 are stopped by a detecting signal of the gates 27, 17, and the electric power source of the additional apparatus 2 or the radio device 1 is turned off.