A light scanning optical system that is capable of increasing a deflection angle without causing optical performance degradation includes a reflection surface for reflecting incident light, a unit for rotating the reflection surface about a swing axis existing in substantially the same plane as the reflection surface, and an optical system that guides light deflected at a first deflection point on the reflection surface to a second deflection point on the reflection surface. The optical system is constructed such that the first deflection point and the second deflection point are in an approximately conjugate relationship.