PURPOSE: To obviate unnaturalness due to zooming on a screen by controlling the moving speed of a movable lens in its optical axis direction so that a focal distance is changed approximately linearly to zooming time.

CONSTITUTION: When a motor 9 is rotated at a constant speed and a cylindrical cam 7 is rotated by zooming drive, a mirror barrel 5 is guided by cam groove 8 together with a movable convex lens L2 and moved in them optical axis direction. When it is defined that the radius of the cylindrical cam 7 is R and a rotational angle θ is expressed by radian, the planely developed shape of the cam groove 8 formed so as to be approximately the same as a curve obtained by expanding a curve expressing relation between the rotational angle θof the cam 7 and a distance (a) R times. At the time of moving the lens L2 by means of the cam groove 8, the focal distance f0 of a zoom lens is approx imately proportioanl to the rotational angle θ of the cam 7 and is approximately linearly changed. When the rotational speed of the cam 7 is fixed, the focal distance f0 is approximately linearly changed in accordance with the elapsed. Consequently, an unnatural change due to zooming in the size of the screen can be obviated.