PURPOSE: To efficiently draw even a straight line, whose inclination is about 45°, with a low frequency in calculation by using 45° segments to draw the straight line.

CONSTITUTION: Conditions related to the straight line to be drawn, namely, the start point coordinate, the limit point of drawing, and the inclination of the straight line are inputted to a straight line drawing part 2. The part 2 obtains two kinds of segment length as the reference and displays the first 45° segment having the start point coordinate as the start point on a bit map display device 1. The start point of the segment to be next drawn is obtained, and the error extent between the drawn segment and a virtual straight line and the variation of the error extent of two kinds of segment length are obtained. When it is judged thereafter that the start point of drawing of the next segment does not exceed the limit point and the initial value of the error extent is smaller than 0, another 45° segment is drawn from the start point of drawing, and the next start point of drawing is obtained and the error extent is updated by the variation of the pertinent segment length. Drawing is repeated in this manner till it is judged that the start point of drawing of the next segment exceeds the limit point of drawing.