A compact, bright and high-performance viewing optical system and an image display apparatus using the same. The viewing optical system uses an ocular optical system, which is formed from a decentered prism, and a reflection type image display device. The image display device has a light source and an illuminating light guide prism for guiding a light beam from the light source so that the light beam is applied to the display surface from the front side thereof. The ocular optical system includes a prism having an entrance surface through which a light beam reflected from the display surface enters the prism after passing through the illuminating light guide prism. The prism further has a reflecting surface and an exit surface through which the light beam exits from the prism. The reflecting surface is decentered with respect to the optical axis and has a rotationally asymmetric curved surface configuration that corrects decentration aberrations due to the decentration of the reflecting surface and gives a power to the light beam. The spacing between the entrance surface of the prism and the display surface satisfies a predetermined condition to lead the image to the pupil position.