The present invention relates generally to a projector apparatus, and more specifically to a projector apparatus incorporating a prism optical system of positive refracting power. 
So far, there have been projector apparatus known in the art, in which size reductions are achievable by use of a prism optical system having a concave internal reflecting surface for internal reflection of normal light beams and a combined optical surface located at a position opposite to that internal reflecting surface and having a combined reflection/refraction action. 
In particular, Patent Publication 1 discloses a projector apparatus in which a stop member adapted to determine the thickness of a light beam emitted out of the center of an image display plane and projected while its periphery is shielded off is located on a projection side with respect to the exit surface of a prism optical system.  This makes improvements in telecentric capability on the image display plane side, and prevents extraneous light from entering the prism optical system and shields off inessential light. 
Patent Publication 1: JP(A) 2003-43361
In the prior apparatus, however, the distance between the prism optical system and the image display plane is short.  In such a prism optical system as set forth in Patent Publication 1, accordingly, a light beam that should be refracted through the combined optical surface and a part of a rim light beam around it are reflected off, turning into inessential light.  As this inessential light is again reflected off at the image display plane to enter the prism optical system for projection, it causes a worsening of the quality of a projection image, and gives rise to flares around the projection image. 
In addition, when the image display plane of a reflection type display device is used as the image display plane, the angle of illumination light on the image display plane grows too large to make sure an optical path taken by illumination light for lighting the image display plane with high efficiency.