An optical beam scanning apparatus according to the present invention includes a light source, a pre-deflection optical system, a light deflecting device, a post-deflection optical system configured to at least include one or plural first optical elements which act on the luminous fluxes for all color components, plural second optical elements which respectively act on the luminous fluxes for each of color components, and plural first reflection mirrors that are respectively provided on an upstream side of the plural second optical elements in optical paths and reflect luminous fluxes emitted from the first optical elements, and a position adjusting mechanism configured to adjust positions of the first reflection mirrors.