PURPOSE: To prevent a ghost light beam without any machining process wherein a groove wall is painted in black by forming a V-shaped groove instead of a conventional U-shaped groove.

CONSTITUTION: A light beam 2 which is made incident from the light-beam incidence surface 10 of the right-angled prism and shown by a broken line is reflected by a 1st reflecting surface 30 and reflected totally by the slanting surface 50 of the V groove on the side of the incidence surface 10, so the light does not exit from the projection surface 20. However, there is the possibility that totally reflected ghost light is reflected repeatedly in the prism and exits from the projection surface 20. The angle (a) of the slanting surface 50 of the V groove on the side of the incidence surface 10 to the incidence surface 10 is so set that 35° < a < 45°, thereby adjusting the direction of the ghost light. When the angle (a) is larger than the upper limit, light 2b is reflected by the slanting surface 50 of the V groove on the side of the incidence surface 10 and reflected by the 1st reflecting surface 30 as shown by 2b'. Then a light beam which travels toward a 2nd reflecting surface 40 is reflected here and exits from the projection surface 20 eventually. When the angle (a) is smaller than the lower limit, on the other hand, the groove becomes shallow and the effect of ghost light removal is reduced.