A manufacturing method of an optical device includes an optical component housing operation S1 for housing a plurality of optical components in a casing body, an optical component positioning operation S2 for adjusting the positions of the optical components by shifting the optical components to position by positioning members at predetermined positions on an illumination optical axis of a light beam irradiated by a light source, and an optical component position fixing operation S3 for fixing the positions of the optical components on the casing body after the optical component positioning operation S2. The optical component housing operation further includes a positioning member inserting process for inserting the positioning members to holes on the casing body, an optical component housing process for housing the optical components in the casing body, and a positioning member abutting process by which the positioning members abut on the optical components.