An optical beam scanning device is configured from a deflecting unit that has a function to support a deflecting member and a light source, including an exposing semiconductor laser, a polygon mirror, a first f.theta. lens and a second f.theta. lens, and strictly regulate their position relative to each other; and a support member that has a function to support a mirror, a second cylindrical lens and a AD sensor along with the deflecting unit, and appropriately position them relative to a photoreceptor drum; and a housing that has a function to cover the outer portion of the deflecting unit and the support member, along with strengthening the support member.