PURPOSE: To prevent the wrong key operation and to improve workability with a display device for key switch function, by pushing light the function keys to confirm that the function with which the display is changed is equal to a desired one and then pushing strong the function keys for execution of functions.

CONSTITUTION: A function D, for example, which is set at the 4th position from left is executed among those functions displayed on a display means 1. Thus a key set at the 4th position from left is pushed light among function keys 4. Then a contact 5 conducts and therefore a control means 2 detects that the 4th key is pushed light and displays adversely the function D. Then the 4th key is pushed strong after confirming that the function D is displayed adversely. Thus a contact 6 conducts and therefore the means 2 detects that the 4th key is pushed strong to execute the function D.