PURPOSE:To secure positioning, by providing a driving device which drives a positioning member so as to move either of a couple of positioning member on a contacting part movement path selectively at the change of magnification. CONSTITUTION:An arm 17 is held at an intermediate position at the change of magnification, and a supporting body 11 is slid along a shaft 13; if a top position corresponding to set magnification is given by, for example, a positioning member 14b when an optical element 10 comes closer to the position corresponding to the set magnification, an actuator 18 moves the arm 17 to an uppermost position. Then, while parallel relation to the movement path P is maintained, the positioning plate 14 is moved toward the movement path P, and positioning part 14b is placed on the movement path, and a pin 12 is allowed to abut on its abutting surface, performing the positioning.
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