There are provided a compact light scanning apparatus including a deflection unit for deflectively scanning light fluxes, and an imaging optical systems provided for the respective light fluxes. When the light fluxes deflectively scanned by the deflection unit are focused onto different photosensitive drums by the imaging optical systems, through a mirror. One of the imaging optical systems includes a transmission type imaging optical element. In a sub-scanning section, the principal ray of a light flux passing through the transmission type imaging optical element passes a side opposite to the optical path of the light flux deflectively scanned by the deflection unit and traveling toward the mirror of another imaging optical system among the imaging optical systems with respect to a straight line connecting the surface vertex of the incidence surface of the transmission type imaging optical element and the surface vertex of the emergence surface thereof.