To provide a laser exposing device in which a laser light source is rationally arranged while taking the abilities of red, green and blue laser light sources into consideration.

This laser exposing device 100 is provided with the red laser light source 10, the green laser light source 11, and the blue laser light source 12 for forming a color image on a photographic paper 1 with the laser beam, a polygon mirror 30 and an fθ lens 32 for performing scanning with the laser beam on the paper 1. In this case, this device is provided with a synthesizing means C synthesizing the respective laser beams from the light sources 10, 11, and 12, and the blue laser beam is synthesized on the most downstream side of an optical path.