There is provided a scanning optical system, which is provided with a deflector that deflects the plurality of beams, and an imaging optical system that converges the plurality of beams deflected by the deflector to form a plurality of beam spots on surfaces to be scanned, respectively. The imaging optical system has at least one lens whose position is changeable in a plane including an optical reference axis thereof and parallel with the main scanning direction. In this configuration, the imaging optical system satisfies a condition: 0.05