An image forming apparatus comprises a signal generator generating a clock signal associated with image forming in a main-scanning direction, an image forming unit including a write unit performing the image forming in the main-scanning direction on a paper relatively moving in a sub-scanning direction, in synchronization with the clock signal generated at the signal generator, an image processor converting input image data into data for driving the write unit, a magnification setting unit setting a magnification of an image in the main-scanning direction associated with a position of the image in the sub-scanning direction, a controller controlling to generate the clock signal of a frequency corresponding to a relative position in the sub-scanning direction on the basis of the magnification set at the magnification setting unit when the write unit performs the image forming on the basis of the data for driving converted at the image processor.