Abstract: A stereoscopic projecting apparatus has a polyhedral prism and two types of polarizing filters which are arranged at the incident (or emerging) planes of the prism and which have orthogonal deflecting directions. Left and right-eye pictures are alternately formed on the consecutive frames of a cinefilm. The cinefilm is driven in such a way as to cross the projection region of light emitted from a lamp. An optical image transmitted from the cinefilm is projected onto a screen by the polyhedral prism, through a relay lens, a projection lens or the like. The left and right-eye images are deflected in directions 90.degree. apart from each other, by the polarizing filters at the respective incident or emerging planes of the prism. A viewer can stereoscopically observe the image superimposed on the screen, using polarized viewing glasses having polarized lenses deflected in the deflecting directions of the images.