In an optical scanning device employing a multi-beam scanning method, a surface emitting laser light source includes laser emission sources; a parallel-plate-like quarter wavelength plate is arranged between the surface emitting laser light source and a first optical system; a light-intensity detecting unit separates light intensity of the laser beams of which form is converted by the first optical system and detects separated laser beams; and a light-intensity adjusting unit adjusts emission intensity of the laser emission sources individually based on a detection result by the light-intensity detecting unit. The quarter wavelength plate is arranged so that an optical axis thereof is tilted ±45 degrees with respect to the main-scanning direction around an optical axis of the first optical system.