The main object of the present invention is to present a projection system excellent in visibility from inside of projection system to outside, and capable of obtaining a sharp contrast display without being influenced by the ambient light.The present invention provides a projection system comprising: a projection screen, containing a substrate and a polarized light selective-reflection layer formed on the substrate and diffuse-reflects right-handed circularly polarized light or left-handed circularly polarized light, which displays an image by reflecting an image light irradiated from a projector; a projector for projecting an image light on the projection screen; and an external light screen installed at an external light entrance, disposed in a sequence of a linearly polarizing plate and a retardation plate from an external side, and circularly polarizes a light in reverse direction to the circularly polarized light diffuse-reflected by the polarized light-selective reflection layer, wherein a transmission axis of the linearly polarizing plate is in vertical direction.