To prevent temperature rise of a lens barrel to prevent deterioration of optical performance of an optical device even if the lens barrel holding a lens within a projection lens unit is exposed to unnecessary light of projected light.

A light beam emitted from a color composition prism 14 is transmitted by lens groups L6-L11 and reaches a forward projection screen. An effective light beam emitted from the color composition prism 14 is prevented from directly reaching an inside diameter part 38c of a cam cylinder 48 by an aperture diaphragm 43. Since a fixed diaphragm 44 is provided, the reflected light by the inside diameter part 38c of the cam cylinder 48 is shielded by the periphery of the fixed diaphragm 44 so as not to reach the screen, and the shielded luminous flux is shown in the drawing by an alternate long and short dash line.