An optical scanning device includes: a light source including a plurality of light-emitting elements; a deflector that defects light beams output from the light source; a scanning optical system that condenses the light beams deflected on the deflector onto a surface to be scanned, and includes at least one resin scanning lens and at least one folding mirror disposed behind the at least one resin scanning lens; a light-receiving element to which part of the light beams, which is deflected on the deflector but not used for scanning the surface, enters not via the at least one folding mirror as light-amount monitoring light beams; and a controller that controls a driving signal for the light-emitting elements based on an output signal from the light-receiving element.