An optical device includes a drive unit configured to drive a light source that outputs a laser beam; a detecting unit configured to detect the laser beam output from the light source; a converting unit configured to convert an output of the detecting unit into a value within a predetermined range; and a control unit configured to control the drive unit to switch a light quantity of the laser beam output from the light source from a first light quantity within an imaging light quantity range for forming an image to a second light quantity outside the imaging light quantity range, or vice versa. The converting unit converts an upper limit light quantity in the imaging light quantity range into a maximum value in the predetermined range, and converts a lower limit light quantity in the imaging light quantity range into a minimum value in the predetermined range.