A scanning exposure device includes: a reflecting member that reflects a light irradiated by an irradiating unit; a housing that has a bottom, a first sidewall, a second sidewall, and an aperture opening in a direction of a light reflected by the reflecting member; a cover that covers the aperture, forms a passing portion that allows the light reflected by the reflecting member to pass through, and provides a first fixed position at which the housing is fixed on the first sidewall and a second fixed position, paired with the first fixed position, at which the housing is fixed on the second sidewall, wherein the first fixed position and the second fixed position are arranged such that the main scanning direction passes between them.