To capture image data through an external I/F without taking time for transfer and to execute output of high grade.

Binary image data of 300 dpi are inputted to an image processing part 1 from a personal computer PC through the external I/F. A resolution enhancing circuit 3 enhances a resolution to 600 dpi by adding novel lines in the sub-scanning direction. A smoothing circuit 5 executes smoothing by using a plurality of filters so as to determine binary data of pixels which are suspended from conversion into the binary data and of which the density is unknown, out of pixels contained in the novel lines added by the circuit 3. A data synthesizing circuit 4 synthesizes an output of the resolution enhancing circuit 3 and an output of the smoothing circuit 5 and outputs binary printing data for making a stencil paper to a plate making part 61 via a data selecting circuit 6.