in fig5 while this specific embodiment has been described and illustrated as the linear driving means used to move first driving lens barrel 22 relative to stationary lens barrel 21 comprising a combination of the rotatable driving screw 25 and nut member 24 threaded with driving screw 25 it is possible to employ another mechanism as this linear driving means for example the linear driving means may comprise a rack formed on a portion of first driving lens barrel 22 and a gear adapted to be engaged with the rack so that the gear may be rotated by electric motor 6 specifically so long as first driving lens barrel 22 can be received by stationary lens barrel 21 and rotatably support second driving lens barrel 27 first driving lens barrel 22 can be provided with a desired component or partially cut off to obtain a desired configuration because first driving lens barrel 22 is linearly driven in the direction of the optical axis s without rotation around the optical axis s referring now to fig7 and 8 an alternative embodiment of the mechanism adapted to guide the back and forth movement of movable lens barrel 30 is described it should be understood that the parts similar to those in the embodiment shown by fig1 through 6 will be.