The light scanning optical apparatus for reducing registration displacement during a temperature increase, includes: multiple light source units; a common optical deflector deflectively scanning light fluxes emitted from the multiple light source units; and first and second imaging optical systems facing each other across an optical direction, sandwiching the optical deflector, and guiding the light fluxes from the optical deflector to different scanned surfaces; and a housing member housing each of the first and second imaging optical systems. Among imaging optical elements constituting those imaging optical systems, two imaging optical elements disposed adjacently to the optical deflector include main scanning direction standard positioning portions disposed at different positions in a scanning direction in those imaging optical systems. The main scanning direction standard positioning portions are positioned with respect to a standard reception portion of the housing member.