Scanning optical system having a fall-down correcting function

A scanning optical system having a fall-down correcting function, comprising:

• a light source portion;
• a first imaging optical system for linearly imaging the light beam from said light source portion;
• a deflector having its deflecting-reflecting surface near the linear image formed by said first imaging optical system;
• and
• a second imaging optical system disposed between said deflector and a medium to be scanned, having means for making constant the moving velocity of a light beam spot on said medium, and comprising, in succession from the deflector side, a spherical single lens and a single lens having a toric surface, said toric surface having its curvature radius, in the deflecting surface formed by the light beam deflected by said deflector, greater than its curvature radius in the plane orthogonal to the deflecting surface, said single lens having a toric surface having a positive refractive power and comprising a surface having a negative refractive power on the deflector side and a surface having a positive refractive power on the medium side, in a cross-section containing the optical axis of said spherical single lens and perpendicular to the deflecting surface, said single lens having a toric surface further satisfying (1/r.sub.3)>(1/r.sub.4), where r.sub.3 is the curvature radius of the deflector side surface thereof in said deflecting surface and r.sub.4 is the curvature radius of the scanned medium side surface thereof in said deflecting surface.