To provide an optical scanner which is made compact, without increasing the cost, and an image forming apparatus.

The optical scanner 16 includes: a deflector (polygon mirror 19) which deflects and scans a light beam emitted from a light source (laser-light emitting part 100); an imaging optical system (fθ lens 20, compensating lens 21, or the like) which images the light beam deflected by the deflector onto a face to be scanned F; a photosensor 102 which detects the light beam; and a first reflecting mirror 101, which reflects the light beam which runs off an effective scanning region A used for image formation among the scanning region of the light beam onto the optical sensor 102. The first reflecting mirror 101 and the photosensor 102 are disposed, in such a way that the light beam reflected on the first reflecting mirror 101 reaches the photosensor 102 passing through the effective scanning region A1 of the lens (compensating lens 21) which has positive refractive power among optical elements that constitute the imaging optical system.