Abstract: This invention concerns an optical system for a projection display device that includes: a first lens which may emit light entering from a light source with a same angle to a single point; a second lens which is positioned in a way that the light collected from the first lens may be transmitted to a digital micromirror device (DMD) with a predetermined angle; a DMD which may reflect the light entering from the second lens; a prism positioned between the second lens and the DMD that transmits and transfers the light emitted from the second lens to the DMD, and transfers the light reflected from the DMD to a projection lens by totally reflecting the light; and a projection lens that is positioned in the optical path of the light totally reflected from the prism, to project the light onto a screen.