The scanning optical device (6) includes a light source, a deflector (30) configured to rotate on a horizontal surface around a rotating shaft extending in a vertical direction and an optical box (50) in which the light source, the deflector (30), and the optical components groups (40) are housed and supported by the frame (60). A heat sink (36) is disposed between the deflector (30) and the frame (60). A first protrusion (37) extending on an extension line of the rotating shaft of the deflector (30) is formed in the heat sink (36). A first engaging hole (61) with which the first protrusion (37) engages is formed in the frame (60).