A scanning optical device, comprising: light source means; a first optical system configured to collect a light beam emitted from said light source means; deflecting means configured to scanningly deflect the light beam from said first optical system; a second optical system having at least one imaging optical element made of a resin and configured to image the light beam scanningly deflected by a deflecting surface of said deflecting means onto a surface to be scanned; characterized in that the scanning optical device further comprises a third optical system configured to divide a light beam outside an image forming region scanningly deflected by the deflecting surface into a plurality of light beams with respect to each of a main-scan direction and a sub-scan direction; detecting means configured to detect the plurality of light beams divided by said third optical system in each of the main-scan direction and the sub-scan direction, an arithmetic unit configured to determine 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 on the basis of positional information concerning a spacing in the main-scan direction between imaging positions of a plurality of light beams and a spacing between the imaging positions in the sub-scan direction of the plurality of light beams as detected by said detecting means; and a driving device configured to move, based on the determination by the arithmetic unit, optical elements (2, 4) of said first optical system in an optical axis direction in order to correct the focal shift in the main-scan direction and the focal shift in the sub-scan direction. light source means; a first optical system configured to collect a light beam emitted from said light source means; deflecting means configured to scanningly deflect the light beam from said first optical system; a second optical system having at least one imaging optical element made of a resin and configured to image the light beam scanningly deflected by a deflecting surface of said deflecting means onto a surface to be scanned; characterized in that the scanning optical device further comprises a third optical system configured to divide a light beam outside an image forming region scanningly deflected by the deflecting surface into a plurality of light beams with respect to each of a main-scan direction and a sub-scan direction; detecting means configured to detect the plurality of light beams divided by said third optical system in each of the main-scan direction and the sub-scan direction, an arithmetic unit configured to determine 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 on the basis of positional information concerning a spacing in the main-scan direction between imaging positions of a plurality of light beams and a spacing between the imaging positions in the sub-scan direction of the plurality of light beams as detected by said detecting means; and a driving device configured to move, based on the determination by the arithmetic unit, optical elements (2, 4) of said first optical system in an optical axis direction in order to correct the focal shift in the main-scan direction and the focal shift in the sub-scan direction.