A scanning optical apparatus includes scanning units, a deflector. Each of the apparatuses includes a light source, an imaging optical system including imaging optical elements imaging a beam deflected by the deflector on a scanning surface. The scanning surfaces are scanned with beams in opposing directions. When a main scanning direction of the first scanning unit is a Y positive direction, a sign is the same between θ1h(Y1min)×θ1r and θ2h(Y1min)×θ2r. Here, θ1h(y), θ2h(y) denote orientation angles between the Y axis and a slow axis at a Y direction position y of first/second imaging optical elements from an optical axis, the imaging optical elements each being an imaging optical element having a largest thickness among them of the imaging optical systems, θ1r and θ2r denote angles between the Y axis and polarization directions of the beams reaching the imaging optical elements, and Y1min denotes a Y position where θ1h(y) becomes minimum.