1. An image processing apparatus, comprising: an input unit successively inputting a first image signal representing density level of each pixel; a thresholding unit generating a second image signal by comparing the first image signal input from said input unit with a threshold value; a calculating unit calculating, based on the second image signal generated by said thresholding unit and the threshold value used for generating the second image signal, a threshold value to be used for thresholding a succeeding pixel; and a changing unit for enlarging or reducing at least one of a range of the first image signal input from said input unit and range of the threshold value calculated by said calculating unit.