To enable to be attached to a nearly round shaped display unit without a useless space, and display the unit in a similar way of visibility even in a dark situation in which external light is not available compared with a bright situation in which the external light is available.

An LED 13 is turned on in the dark situation in which the external light cannot be obtained. This light from this LED 13 is transmitted inside a light guide plate 11 (arrow head A). The light which reaches a side face 11c is reflected at the side face 11c and transmitted again inside the light guide plate 11 (arrow head B). The light reflected at the side face 11c has an optical course changed at a steep slope 12a of a prism 12, emitted to a display plate 20 from a light outgoing face 11a, and illuminates the display plate 20 (arrow head C). The light which illuminated the display plate 20 is reflected at the display plate 20, penetrates through the light guide plate 11 and a protection glass plate 21, and reaches eyes of an observer (arrow head D).