To provide illuminating device in which the valid use of an illuminating light can be attained, and a reflected light on the edge of a protruding part formed on a light transmission plate can not be seen by an observer by allowing the protruding part to have a face which is almost perpendicular to the traveling direction of an incident light from the light transmission plate, and allowing a boundary between the root of the protruding part and the light transmission place to have an inclination other than a right angle to the incident light.

A protruding part formed on a light transmission plate has a face which is almost perpendicular to the traveling direction of an incident light from the light transmission plate, and a boundary between the root of the protruding part and the light transmission plate has an inclination other than a right angle to the incident light. A twist-shaped protruding part having a top face(the top end side face of the protruding part) 11a at a twisting position to a bottom face (the root side face of the protruding part) 11b is projected like a column with prescribed pitches on the back face of the light transmission plate. In this case, it is desired that an angle θ made by the bottom face 11b and the top face 11a is ranging from 20 to 70°. Also, in this protruding part, the bottom face 11b is formed on the light transmission plate so that the incident angle of an illuminating light from a light source to an edge 11d can be prevented from being 0°, that is, an incident angle β of the illuminating light can be a positive acute angle.