An optical beam scanning apparatus according to the present invention includes a pre-deflection optical system, a post-deflection optical system at least including one or plural first optical elements, plural second optical elements, a first reflection mirror which is provided in optical path between the first optical element and one of the second optical elements and reflects, on a most upstream side of the optical paths, a luminous flux on a most downstream side or a most upstream side in the sub-scanning direction among the plural luminous fluxes, and a second reflection mirror which is provided in optical path between the first optical element and another second optical element and reflects, on an optical path second from a most downstream side of the optical paths, the luminous flux on the most downstream side or the most upstream side in the sub-scanning direction among the plural luminous fluxes.