Provided is an image writing device including a deflector having deflective reflection surfaces for deflecting light flux emitted from a light source and a scanning imaging optical system that condenses the light flux as a light spot on a scanned surface of a latent image carrier, and performing optical scanning on the scanned surface at a constant speed, the image writing device further including: a surface detector that detects a deflective reflection surface that deflects the light flux; a storage that prestores a beam irradiation position in a sub scanning direction corresponding to each main image height on each of the deflective reflection surfaces; and a hardware processor that controls, on the basis of a beam irradiation position in the sub scanning direction corresponding to each main image height on the deflective reflection surface a light quantity of the light flux to be irradiated to the beam irradiation position.