an optical apparatus includes a laser beam emitter for outputting a laser beam modulated in accordance with image information , and a lens unit resting on a lens mount . the lens unit includes , all formed integral with one another , an effective lens portion , a cylindrical portion surrounding a side surface of the effective lens portion , and projections extending from peripheral positions of the cylindrical portion . the lens mount includes a holder for holding the cylindrical portion of the lens unit to be rotatable and axially slidable on an optical axis of the laser beam , and a groove formed in the bottom of the holder for receiving the projection .