An image forming apparatus includes an input unit for inputting image data and a converter for converting the input image data into dot information. An image forming circuit controls a scan optical system on the basis of the dot information to form an image. The converter converts the image data into dot information having a screen angle by repeatedly using a predetermined threshold matrix so that the dot information appears preferentially in a sub-scan direction, perpendicular to the main-scan direction of the scan optical system. In the threshold matrix, threshold values are arranged so that their levels increase preferentially in the sub-scan direction.