To obtain a multibeam optical scanning optical system and an image forming apparatus using the same capable of forming a high-quality image at high speed.

The scanning optical system is provided with a first optical system for changing each luminous flux emitted from a light source means including a plurality of light emitting parts to convergent luminous flux or divergent luminous flux, a second optical system for forming an image in a long line state with each luminous flux emitted from the first optical system, a light deflector having deflection surfaces in the vicinity of a plurality of line image forming positions and deflecting each incident luminous flux so as to perform scanning, and a third optical system for forming the image on a surface to be scanned with each luminous flux deflected by the light deflector and also making the deflection surface of the light deflector and the surface to be scanned have nearly conjugate relation on a subscanning cross section. A relay optical system is arranged between the light source means and the first optical system, and the main light beam of each luminous flux made incident on the surface to be scanned and the normal of the surface to be scanned form a specified angle in a subscanning direction.