A scanning optical apparatus includes: a light source; a first optical element configured to convert light emitted from the light source into a beam of light; a second optical element configured to convert the beam of light having passed through the first optical element into a linear image extending in a main scanning direction; a deflecting mirror configured to deflect the beam of light having passed through the second optical element in the main scanning direction; and a third optical element configured to convert the beam of light having been deflected by the deflecting mirror into a spot-like image and focus it on a target surface to be scanned. The third optical element is a single lens having a pair of opposite lens surfaces, and each of the pair of lens surfaces is aspheric in a main scanning plane to satisfy the formula:  ( 1 - S k  ( y 1 , y 2 ) f t  ( y 1 , y 2 ) ) · h  ( y 1 , y 2 ) 