An image processing system which partitions an input image data into pixel density data representing a density of each of pixels of the input image and stores the pixel density data. The pixel density distribution of the input image is then computed by counting pixels of each density represented by the stored pixel density data. Accumulation data is then generated corresponding to each density represented by the pixel density data, and a data curve is generated and output representing accumulation data. The data curve representing accumulation data corresponding to each density represented by the pixel density data obtained from the input image data is read and used to correct the converted pixel density data by employing the data curve as a density correction curve indicating a relation between the pixel density data and a density data output. Further, the accumulation data corresponding to each density represented by the pixel density data represents a value proportional to the number of pixels having densities ranging from the lowest density to a density corresponding thereto.