As a light beam of a scanning optical system unit is adjusted in the state where the scanning optical system unit is assembled, its adjustment work and the assembly work of the scanning optical system are prevented from being useless. Thus, a scanning optical system adjusting device which can adjust the scanning optical system quickly and at a low cost is to be provided. A scanning optical system adjusting device according to the invention includes a static angle control unit configured to maintain a rotation angle of a rotary polygon mirror in the assembly unit of the scanning optical system at a predetermined angle and make the rotary polygon mirror static there, and a light beam parameter measuring unit configured to measure a predetermined parameter of a light beam reflected by the rotary polygon mirror maintained at the predetermined angle by the static angle control unit.