PURPOSE: To prevent the power source of a controller from being turned off carelessly by connecting the contact of a relay in parallel to the contact of a power-off switch and controlling the state of the contact of the relay according to whether a password is inputted or not.

CONSTITUTION: The power source is turned on and then the contact 24a of the relay connected in parallel to the power-off switch 13 is held on. Therefore, even if the switch 13 is turned off carelessly in this state, the power source never turns off. When the power source is turned off, on the other hand, an operator inputs the predetermined password on a keyboard 22 firstly. Consequently, a microprocessor 18 sends a control signal to an input/output part 20 to control a relay control circuit 23 and stops the supply of a working current to the relay 24, thereby breaking its contact 24a. Then, the operation of the switch 13 becomes effective.