PURPOSE: To cut off external disturbing light and to improve the visibileness of necessary light from a display device by polarizing external incident light, and arranging a polarizing plate, which allows only polarized light perpendicular to the incident light to pass through it, on the front surface of the display device.

CONSTITUTION: External disturbing light 3 is polarized by the 1st polarizing plate to strike a display device. In the figure, 6 is the 2nd polarizing plate arranged on front surface of the display device. When the polarizing plate 6 is so arranged that the plane of polarized light passing through this 2nd polarizing plate is at right angle to the plane of polarization of the incident light, the disturbing light incident to the display surface of a measuring instrument is cut off. Practically, the plane of polarization is rotated in such a way that reflected light from the display surface of the measuring instrument is minimized.