To provide a scanning optical device most suitable for a high image quality image forming apparatus without generating light and shade irregularity on an output image by reducing pitch shift of a rotation cycle of a polygon mirror.

The scanning optical device includes a light source device for emitting a laser light flux, a cylindrical lens for regarding the laser light flux emitted from the light source device as a converging light flux in only a sub-scanning direction, a deflection means for deflecting the laser light flux passing through the cylindrical lens, and an image forming means for image-forming the laser light flux deflected by the deflection means on a face to be scanned. The scanning optical device comprises a rotating polyhedral mirror and a rotating means having a seat face installing the rotating polyhedral mirror and rotating it. The center of the rotating polyhedral mirror is eccentrically installed in an inclination direction of a normal line of the seat face.