PURPOSE: To judge easily whether the display is unavailable or not by inputting preset numerical information or putting a character key on and displaying a display character on a display.

CONSTITUTION: When the numerical information is input from a keyboard 11 by a numeric key 11a, a first storing section 13 is retrieved by a controller 12 to check whether any invalid numerical data X corresponding to the numerical information is stored or not. In the case the invalid numerical data X is stored, that is, when a numerical value X is input from the keyboard 11, a display character code X corresponding to the invalid numerical data X is read out by the controller 12 and transferred to a display 15. Display characters are displayed on the display 15 by said process. On the other hand, when a character code key 11b is pushed, the invalid numerical data X is read out from a second storing section 14 by the controller 12, and the display character code X corresponding to invalid numerical data 13 is read out and transferred to the display 15.