A scanning optical system capable of sufficiently enlarging an exit pupil and being reduced in size is disclosed. The scanning optical system has a scanning device used to scan light from a light source and a first optical system used to directs the light scanned by the scanning device to an exit pupil. The first surface has at least a reflective action and is decentered with respect to a central principal ray. The second surface reflects the light reflected by the first surface again toward the first surface. The first surface reflects the central principal ray, which has again been made incident to the first surface from the second surface, toward a side substantially opposite to a side toward which the central principal ray was reflected the last time with respect to a normal on a hit point of the central principal ray in the first surface.