PURPOSE: To plot a natural straight line and a polygonal curve at a high speed, and to attain a satisfactory graphic plotting by plotting a prescribed straight line by matching a round cap with the start point and end point of the straight line.

CONSTITUTION: This device is equipped with a host computer 1 being an outside inputting device, printer 2, and printer controller 3. The printer controller 3 receives print data outputted from the host computer 1 through a data inputting part 4, searches and corrects four coordinates defining the straight line by an image data expanding part 5, stores a result obtained by painting-out the inside in a print image storage part 6, and transmits the print image data through a data transferring part 7 to the printer 2. Then, the straight line is plotted by matching the round cap with the start point and end point of the straight line. Therefore, the normal straight line whose width is more than 2 dots can be plotted so as to be equally thickened from a central line combining the start line with the end line, while the round cap and the both edge parts of the straight line can not be shifted.