To provide a projection type display device capable of improving light utilizing efficiency in a projection optical system for correcting trapezoidal distortion generated in an intermediate image by oblique projection and capable of projecting a large screen of an extra-thin type in a fold-back projection system by a plurality of reflectors.

In this projection optical system, trapezoidal distortion generated in an intermediate image by oblique projection, an optical deflection means for matching or nearly matching the optical axes of optical systems in front/rear of the intermediate image with each other is constituted. In this projecting display device for folding back a luminous flux form a display element with a reflector to project on a screen, a projection lens and plural reflectors arranged vertically or nearly vertically are constituted in an order from the side of the display element.