An image forming method including: inputting a pulse width modulated signal into a laser beam output section and thereby outputting a laser beam, wherein a pulse width of the pulse width modulated signal is changed based on image data containing density information; scanning the laser beam onto a surface of a photoreceptor through an optical scanning apparatus and thereby outputting an image; detecting a diameter of the laser beam in a sub-scanning direction formed on the surface of the photoreceptor, the diameter of the laser beam inherent to each optical scanning apparatus and changing along an image height in a main scanning direction; and correcting the pulse width in response to the diameter in the sub-scanning direction so that the image output density is uniform with respect to one and the same input density in the image data.