An scanning optical apparatus of present invention converts plural light beams emitted from corresponding light source units by a light beam conversion unit, deflectively scans by a deflection unit, focuses by an imaging optical unit onto corresponding scanned surfaces. At least two of light source units are arranged along a direction perpendicular to the direction in which the light beams are emitted. The light beam conversion unit includes plural light beam conversion elements that reflect the light beams emitted in the same direction from the light source units to deflect the light beams in the same direction. Each of the light beam conversion elements has at least one reflecting surface having a power and at least one diffracting surface having a power, and has different powers with respect to the main scanning direction and with respect to the sub scanning direction.