A scanning optical device including: a light source unit; a first optical element that inputs a light flux emitted from the light source unit to output the light flux; a second optical element that converts the light flux emitted from the first optical element into a longitudinal linear image in a main scanning direction; a deflection element that deflects the light flux emitted from the second optical element for scanning; a third optical element that guides the light flux deflected by the deflection element to a surface to be scanned; a synchronous detection element that obtains a synchronous signal; and a fourth optical element that guides the light flux from the deflection element to the synchronous detection element, in which the second optical element and the fourth optical element are independent of each other; and in the case where a point at which a main light beam traveling toward a scanning center on the surface to be scanned is deflected by the deflection element is assumed as a reference point, the second optical element is located at a position which is farther from the reference point than the fourth optical element.