An image processing apparatus capable of reducing a nonuniform pitch in the sub scanning direction of an image, caused such as by unstable paper feed of a printer, for example, is formed as described below. An adder adds the density value of one pixel of an input multi-valued image and a diffused error output from an error weighting filter. A comparator compares an output from the adder and a corresponding threshold of a variable threshold matrix, and outputs the result as a binary image. The variable threshold matrix is formed of two sub matrices with a size of 3×3 connected in a main scanning direction, and an average value of thresholds in one sub matrix is larger than another. Accordingly, a line component vertical to the main scanning direction is added in an output image. This line component masks a nonuniform pitch in the sub scanning direction.