PURPOSE: To make it possible to speed up straight line plotting processing time and to improve plotting quality.

CONSTITUTION: The difference of a starting point and a termination point of a straight line to be plotted is calculated. The value of an operator calculating the plotting start dot of the straight line to be plotted is set and the coordinate of a temporary intersected point in the vicinity of a boundary and outside plotting areas is set. Whether the present value of the operator exceeds a reference value or not is decided. Till when it exceeds the reference value, dummy dots are formed by changing the coordinate of the temporary intersected point and the process defining new value as a present value is repeated by updating the present value of the operator. When the present value of the operator exceeds the prescribed value, the coordinate in a direction orthogonal to 1 direction is defined as a plotting start dot by changing the coordinate by 1 dot in the direction toward the end point.