There is provided an image forming apparatus comprising: a scanning unit configured to scan, in accordance with image signals, a photosensitive member with laser light in a main scanning direction at a scanning speed that is not constant; an image signal generation unit configured to generate image signals that are changed such that the faster the scanning speed is, the narrower an image signal width becomes; a clock signal generation unit configured to generate sampling clock signals for sampling the image signals whose image signal width is changed such that the faster the scanning speed is, the shorter a sampling interval becomes; and a count unit configured to count image signals whose image signal width is changed based on the sampling clock signals.