Abstract: An object of the present invention is to provide a projector optical device in which both of an image forming section and a screen are held stationary in their fixed locations and an object image distance is constant, wherein an effect from a variation of optical characteristics caused by the heat, such as a variation of focal distance of a lens in a projection lens system located in the vicinity of a light source, can be eliminated and thereby a clear image in the initial focused condition can be maintained on the screen.

Abstract: An object of the present invention is to provide a thread coupled lens barrel which can be manufactured easily by using inexpensive members and in which both parts in the thread engagement without any gap or back-lash in the thread engagement site can be aligned with high precision along a center axis line or an optical axis. Provided is a thread coupled lens barrel having a thread engagement site, characterized in that an elastic synthetic resin tape is disposed between a female thread and a male thread in said thread engagement site and a state of thread engagement is secured by a fixing screw. Also provided is another thread coupled lens barrel having a thread engagement site, characterized in that an elastic synthetic resin tape is disposed between a female thread and a male thread in said thread engagement site and a thread engaged rotation between said female thread and said male thread relative to each other in said thread engagement site is made possible.