An optical scanning device including a housing, a first carriage carrying a light source and a first mirror, a second carriage carrying second and third mirrors, the second carriage cofigured to move in a sub-scanning direction at one-half of a speed of the first carriage, first and second timing pulleys spaced apart in the sub-scanning direction, a timing belt spanned around the first and second timing pulleys to reciprocate the first carriage in the sub-scanning direction, a drive device to drive the first timing pulley, flat pulleys supported by the second carriage and spaced apart in the sub-scanning direction by a moving distance of the second carriage, a flat belt spanned around the flat pulleys to reciprocate the second carriage in the sub-scanning direction, an engaging member engaging the timing and flat belts to the first carriage, and a fixing member fixing the flat belt to the housing.