PURPOSE: To enable maintaining of necessary power supply even when the switch is opened while an input/output unit is in operation by conducting a relay which is in parallel with a power supply switch by a controlling signal sent out when an input/output controlling device is in an operating state.

CONSTITUTION: When a power supply switch 2 is closed and an input/output device 6 starts inputting/outputting operation, this is detected by a relay controlling circuit 8 basing on the state of an input/output control device 4, and the relay controlling circuit 8 closes a solid state relay 7. This condition is maintained unit 1 the input/output device 6 completes the operation. Even if the power supply switch 2 is opened by mistake under this condition, power is supplied to a power supply device 3 through the solid state relay 7. When all the inputting/outputting operations are completed, the relay controlling circuit 8 detects completion of operation, and opens the solid state relay 7 to cut off feeding of power to the power supply device 3.