To enable expanding a bit map at high speed without preparing a large scale hardware.

A list generating means 3 generates a list provided with information of the start points and the end points of edges constituting the contours of objects to be plotted from printing data 1 which is inputted to an input means 2 at every object to be plotted and a list classificating means 4 classifies the lists into the ones where the edges are intersected and the ones where they are no'. A coordinate point generating device 5 consists of plural coordinate point generating circuits and generating the plural coordinate points expressing the edges from the lists where the edges are not intersected by sequencing them at a time. A coordinate point generating means 6 sequentially generates the coordinate points from the lists with the intersected edges by executing a prescribed program in a central arithmetic processor. A sorting means 7 sequences the generated respective coordinate points. A converting means 8 extends printing data by bit map through the use of the coordinate points and outputs it to a printing output means 9.