An optical apparatus includes a zoom optical system including a focusing lens unit, a first cam member that rotates around the optical axis and moves along the optical axis in a magnification-changing operation, the first cam member including a focus cam portion for driving the focusing lens unit, and a speed-changing mechanism that changes the relative rotational speed between the focusing lens unit and the first cam member in accordance with the focal length of the zoom optical system in the magnification-changing operation. The speed-changing mechanism moves along the optical axis in the magnification-changing operation. In addition, the speed-changing mechanism includes a transmission member that is rotatable around the optical axis with respect to the first cam member and that includes a speed-changing cam portion for changing at least one of the rotational speed and the rotational direction of the focusing lens unit.