A scanning optical device that scans a light beam through a relay optical system, the relay optical system comprising: front group lenses arranged at a light source side; rear group lenses arranged at a side of a surface to be scanned; and first and second mirrors arranged between the front group lenses and the rear group lenses and supported by a common supporting member, wherein a normal vector of a mirror surface of the first mirror makes an angle (180−ω) with an incident optical axis from the front group lenses, a normal vector of the second mirror makes an angle α (α≠180×m) with the normal vector of the first mirror, and the supporting member includes a mechanism being linearly movable in a direction which makes an angle (ω−α+90) with the normal vector of the first mirror.