An image forming apparatus, including: an image signal generating portion configured to generate an image signal based on image data; a light source configured to emit a light beam based on the image signal; and a deflection device configured to deflect the light beam in a main scanning direction, wherein the image signal generating portion includes: a mode setting portion configured to set a plurality of modes including a mode in which a value of a pixel of the image data is not converted and a mode in which the value of the pixel in one or more lines is converted, to a plurality of areas into which a scanning area is divided in the main scanning direction, respectively; and a pixel value conversion portion configured to convert the value of the pixel in each mode in accordance with a position of the pixel in the main scanning direction.