PURPOSE: To improve the handling facilitation of a display device which is used for the window service jobs, etc., by provide a conversion display control part which inverts a conversion display screen by 180° with a 180° conversion command.

CONSTITUTION: An input information analyzing part 2 checks and decodes the 180° conversion information instructed from an input part 1 and then outputs the conversion information to a conversion display control part 3. The part 3 inverts the screen information to be displayed by a received 180°C conversion command and sends this control output signal to a visible display part 4 to display an inverted image at the part 4. In such a constitution, the display screens can be inverted when a terminal operator pushes a conversion key as necessary in such a case where two persons are using a display device opposite to each other for the window service jobs, etc. Thus the contents o the display screens can be confirmed with no mistake.