PURPOSE: To switch a power supply control function to a communication controller, to a master system automatically, by providing a failure detection signal or a remote control switching signal from a data processor, if the data processor is in failure.

CONSTITUTION: If a failure of a CPU1 takes place, the CPU1 turns on a failure detection signal line 11, to activate a power supply control switching circuit 5 and to make a signal from a power supply control line 9 ineffective, and a CCP3 of a system B is controlled with a signal control through a data line 10 from the CPU1, CH2, a bus line 8, the CCP3 of a system A. Through the transmission of a signal to a remote control switching signal line 12, the CCP3 of the system B is controlled with the signal control through the data line 10 from the CPU1, the CH2, the bus line 8 and the CCP3 of the system A, and the power supply of the CCP3 of the system B is turned off with the power supply off-signal from the system A at the end of job at night.