An optical outer diameter measurement apparatus includes a rotary device driven to rotate by a motor to convert light emitted from a light-emitting element into scanning light, a collimator lens configured to convert the scanning light into parallel scanning light that is projected toward an object to be measured, a condenser lens configured to condense the parallel scanning light having passed by the object to be measured, a light-receiving element configured to receive light condensed by the condenser lens, a housing that accommodates the rotary device and the collimator lens, and an optical-path cover including, in the housing, two walls extending in a scanning direction of the scanning light and located opposite each other with the scanning light between the two walls.