PURPOSE: To prevent deviation when encoded data are plotted on a plotting surface having a different size from that of the encode data, in an image processor decoding the encoded data.

CONSTITUTION: The run length value of the encoded data including the run length value transmitted from a host device is detected by a run length detection part 4. The size of the plotting surface from a host device 1 is detected by an image processing part 3 and compared with the run length value. When the run length value is smaller, a white blank is inserted in the space of the difference between them.