PURPOSE: To prevent option key input errors through the use of a key protect function by processing the input in response to a valid/inhibit state corresponding to the key and the switching state of a protect switch.

CONSTITUTION: The preset counter at application of power checks the content of a dip switch 4 and starts control operation as the preset counter when the operation mode is set. When the dip switch 4 is set to the setup mode, the setting of the operation function of the preset counter is started. A switching setting of a protect switch is processed by a setting key based on the menu on a display section 3 on an operation panel and the valid/inhibit state corresponding to each key is set to storage means 5, 6. When a key entry is received, the entry is processed depending on the valid/inhibit state corresponding to the key and the switching state of the protect switch to enhance the key protect function by the optional key to prevent mis-operation.