PURPOSE: To stop a controller without affecting the state of an instrument as a controlled system without affecting the operating conditions of this instrument.

CONSTITUTION: Normally, the signal of a sensor 1 is inputted to a controller A and this controller A controls an instrument 21 as the controlled system. When stopping the controller A, the signal of a changeover switch 14 is transmitted to an input switching device 2, output hold logic 19 and coincidence deciding means 11. Then, the device 2 transmits the signal of the sensor 1 to a controller B for backup, and the value of an arithmetic logic 7a (the output value of the controller A) just before the switching of the device 2 is held and transmitted through a transmission bus 15 to the coincidence deciding means 11 by the output hold logic 19. When the signal from the switch 14 is received and the value from the bus 15 is equal with the value of 7b, the coincidence deciding means 11 switches the state of an output switching device 12. Afterwards, this device 12 starts transmitting the output of the controller B to the instrument 21 of the controlled system.