PURPOSE: To keep the surface of a control display panel clear so that an indication thereon can be easily seen, by blowing air by means of a fan onto the surface of the control display panel provided in a control position, so as to remove the dust therefrom, and discharging the dust with the air from a dust discharge port provided at a lower side of the air passage.

CONSTITUTION: Air is blown by means of a fan 13 onto the surface of a control display panel 11 on which a control key is provided. The air with dust removed from the surface of the panel 11 is discharged via an air passage on the rear side of an instrument 16 from a dust discharge port 14. Thus, the surface of the panel 11 can be kept clear so that an indication thereon can be easily seen.