An apparatus for image processing is disclosed. The apparatus is for reducing one or more runlengths by converting binary-format image data for each of black and white runlengths in one direction relative to the data so as to obtain the reduced image data relative to that direction. In the apparatus, when performing an operation of reducing a given one of the runlengths, the operation is performed with reference to one or more runlengths, other than the given runlength, of the same line that have already been reduced.