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, a first sensor configured to detect a part of the luminous fluxes deflected by the light deflecting device, one or plural first optical elements configured to be provided in the post-deflection optical system and act on the luminous fluxes deflected by all the deflection surfaces of the light deflecting device; and a second optical element having positive power in the sub-scanning direction configured to be provided in an optical path between any one of the first optical elements and the first sensor and act on the luminous fluxes deflected by all the deflection surfaces of the light deflecting device.