The present invention provides an optical scanning device that can suppress irregularity of the rotation of a polygon mirror and an image forming apparatus that is disposed with this optical scanning device. The optical scanning device includes: a rotating polygon mirror that rotates and deflects light that has been emitted from a light source; and a plate member that covers one surface of surfaces intersecting an axial direction of a rotating shaft of the rotating polygon mirror, wherein a projecting portion that projects in the axial direction of the rotating shaft away from the rotating polygon mirror is disposed in the plate member.