To provide a mutlibeam scanning optical system and a multibeam scanner using the system capable of performing scanning at high speed with low power consumption without spoiling the uniformity of light quantity on a surface to be scanned.

This multibeam scanning optical system is provided with plural light sources, an optical device 4 advancing beams respectively emitted from the plural light sources nearly in the same direction, a light deflector 25 deflecting the plural beams passing through the device 4, and an image-formation optical system 20 forming the plural beams deflected by the deflector 25 into an image on the surface to be scanned 30. In the optical system, the plural beams made incident on the deflector 25 from the device 4 are made incident on the deflection surface at a specified angle to the surface normal of the deflection surface of the deflector 25 on a subscanning cross section, and made incident on the deflection surface from the center or almost the center of the deflection angle of the deflector 25 on a main scanning cross section.