It is an object of the present invention to provide a multi-beam scanning optical
system, which can reduce relative deviation of a writing position of each light-emitting
portion due to focus deviation of a synchronization detection optical device, and
an image forming apparatus using the same. In the multi-beam scanning optical system
having the synchronization detection optical device including a light source device,
an incident optical device, a deflection device, a scanning optical device and
a slit, each element is set such that relative deviation of a writing position
of each light beam due to deviation of a focus position within a main scanning
cross section is reduced by a first light beam limiting element provided between
the deflection device and the slit and a conditional expression is satisfied.
