An optical writing device writes an image on a photoconductor with light emitted from a light-emitting element array including a plurality of light-emitting elements lined in one direction. The optical writing device includes: a thinning unit that thins pixels in input binary image data depending on a pattern of the image data; a thinning rate setting unit that sets a thinning rate by the thinning unit for each predetermined range of the image data; and a light-emitting time changing unit that changes a light-emitting time of the light-emitting element array for each predetermined range of the binary image data thinned by the thinning unit depending on the thinning rate set by the thinning rate setting unit and a set toner saving rate.