To provide an optical scanning system capable of preventing an undesired luminous flux made incident on a reflection type deflector after passing near a normal laser beam in nearly parallel to the normal laser beam from being made incident on a surface to be scanned, as for a double reflection type optical scanning system.

The optical scanning system is constituted so that the absolute value of an incident angle at the 1st time when the laser beam is made incident on one side (reflection surface) of a polygon mirror 2 is made differ from the absolute value of an incident angle at the second time. Besides, the optical scanning system is constituted so that a shielding member 5 for shielding the undesired luminous flux passing through the surroundings of the 1st mirror M1 after being reflected by the polygon mirror 2 only once is arranged on the optical path of the undesired luminous flux.