An optical scanning device includes a light emitter, a scanner, a first detector and a determiner. The light emitter outputs a beam. The scanner scans within a given area determined in advance with the beam output from the light emitter. The first detector is arranged on an optical path of the beam in the given area and detects the light quantity of the beam. The determiner determines light quantity shortage of the light beam if the light quantity detected by the first detector is below a first threshold set in advance and above a second threshold indicating a light quantity smaller than the first threshold and determines an abnormality in a scanning operation by the scanner or an abnormality of the light emitter if the light quantity detected by the first detector is below the second threshold.