Patent ID: 8223415

Claim:
A scanning optical device that scans a light beam irradiated from a light source through a relay optical system on a surface to be scanned, wherein the relay optical system comprising: at least one front group lenses arranged at a side of the light source; at least one rear group lenses arranged at a side of the surface to be scanned; and a first mirror and a second mirror arranged between the front group lenses and the rear group lenses, and supported by a common supporting member so that a position of the first mirror and a position of the second mirror are relatively fixed with respect to each other; wherein the first mirror is arranged such that a normal vector of a mirror surface of the first mirror makes an angle (180−ω) (unit: deg) with an incident optical axis from the front group lenses, and the first mirror folds an incident light beam from the front group lenses, wherein the second mirror is arranged such that a normal vector of a mirror surface of the second mirror makes an angle α (α≠180×m, here m is an integer) (unit: deg) with the normal vector of the mirror surface of the first mirror, and the second mirror folds an incident light beam from the first mirror toward the rear group lenses, and wherein the supporting member includes a mechanism, the mechanism being linearly movable in a direction which makes an angle (ω−α+90) (unit: deg) with the normal vector of the mirror surface of the first mirror.