An optical scanning device is provided, which includes a casing including a supporting wall supporting a deflector, a first reflecting mirror supporting portion and a second reflecting mirror supporting portion that are opposed to each other across the deflector and extend from the supporting wall and a reinforcing wall configured to extend from the supporting wall, between the deflector and first and second light source units, so as to connect the first reflecting mirror supporting portion with the second reflecting mirror supporting portion, the reinforcing wall including a first through-hole configured such that a first laser beam emitted by the first light source unit toward the deflector and the second laser beam emitted by the second light source unit toward the deflector pass therethrough.