This invention provides an image forming apparatus capable of outputting, without using any special device, an image which is free from changes in density and tint even upon crisscross rotation and exhibits good proportion of a small character. To accomplish this, the pulse width of a laser emission signal is controlled at 16 levels for one dot at 600 dpi, controlling an actual image in the main scanning direction. When adjusting vertical and horizontal line widths, patch data are created by rotating an anisotropic pattern through 0° and 90° (S1 in FIG. 5). Then, a reader/scanner (15) reads each patch as density data (S2 in FIG. 5). A pulse width, that is, line width correction value at which the densities of the patch patterns rotated through 0° and 90° become equal to each other is obtained (S3 in FIG. 5).