In an image forming apparatus, a line width detection unit detects a line width from attribute data inputted from an attribute data input unit, an edge detection unit detects an edge from image data inputted from an image data input unit, and an AND calculation unit calculates an AND of results of these detections. A coefficient storage unit stores a coefficient set for each line width. A line width correction coefficient multiplication unit acquires a correction coefficient corresponding to the detected line width with regard to an edge and the detected pixel and performs a multiplication to thereby correct the line width.