To rationally constitute an exposure device which exposes photographic paper by sending a light beam to the photographic paper from below.

The exposure device is constituted forming a transporting mechanism C for sending the photographic paper P in the horizontal direction in it by setting its photosensitive surface so as to face downward, and by housing an optical system having a mirror 5 and lenses 4, 6 and 7 for reflecting the laser beam L sent from a laser transmitter 2 in the horizontal direction in a scanning direction by a polygon mirror 3 rotating around a vertical axis center Y and diagonally upwardly sending the laser beam L from this polygon mirror 3 into a case 9 in such a manner that the photographic paper P is exposed with the laser beam L diagonally upwardly delivered via an opening S of the case 9.