A driving mechanism is described for a variable-focus photographic lens adapted to move back and forth. The driving mechanism includes a first driving lens barrel movably received within a stationary lens barrel without rotating the first driving lens barrel around an optical axis with a driving force transmitted from an electric motor. The driving mechanism of the present invention provides a so-called three-state extension lens barrel which advantageously eliminates the necessity for incorporation of driving ring members into the stationary lens barrel and/or the first driving lens barrel, as well as the necessity for constructing the stationary lens barrel and/or the first driving lens barrel in multiple structure. Such an arrangement positively contributes to the realization of a thin and compact camera. A portion of the first driving lens barrel is formed with a support block which is, in turn, formed with a guide hole into which a guide rod extending in parallel to the optical axis is loosely inserted. A portion of the support block is formed with a nut holder into which a nut member formed therearound with a female screw is loosely inserted and then a driving screw extending in parallel to the optical axis is inserted into a through-hole of the nut holder so that the driving screw may be threaded with the female screw of the nut member. Rotation of a first driving gear fixed on the driving screw causes the driving screw to be rotated and rotation of this driving screw causes the nut member to move along the driving screw. Consequently, the first driving lens barrel moves in the same direction.