Patent ID: 8027074

Claim:
A scanning optical system, comprising: a light source to emit a light flux with a wavelength of 500 nm or less, a deflecting optical system to deflect the light flux from the light source so as to scan in a main scanning direction on a surface, and a scanning imaging optical system to converge the light flux deflected by the deflecting optical system on the scanned surface, wherein the scanning imaging optical system comprises a first plastic lens provided to adjoin the deflecting optical system and satisfies the following conditional formula: 0.05 ≦NA 1 ·t 1≦1.5 where a numerical aperture of a light flux entering the deflecting optical system in a sub-scanning direction is NA 1 and a distance between the deflecting optical system and the first plastic lens is t 1 [mm]; and wherein the first plastic lens is mainly composed of an alicyclic structure containing resin which contains a repeating unit [1] represented by the following Formula (1): in the Formula 1, R 1 represents a hydrogen atom or an alkyl group having a carbon number of 1 to 20, R 2 to R 11 each independently represents a hydrogen atom, an alkyl group having a carbon number of 1 to 20, a hydroxyl group, an alkoxy group having a carbon number of 1 to 20 or a halogen group.