A scanning optical device wherein, on the basis of positional information concerning a spacing in a main-scan direction between imaging positions of a plurality of light beams passed through resin-made imaging optical elements and detected by a photodetecting device and a spacing between the imaging positions in a sub-scan direction of the plurality of light beams, a focal shift direction and a focal shift amount in the main-scan direction as well as a focal shift direction and a focal shift amount in the sub-scan direction are determined, and wherein an optical element of an input optical system is moved in an optical axis direction based on the determination, to correct the focal shift in the main-scan direction and the focal shift in the sub-scan direction.