PURPOSE: To automatically generate accurate polygonal arc coordinates and to plot a network graphic by specifying the start and end point positions of an arc at the time of plotting a polygonal line arc consisting of horizontal and vertical segments.

CONSTITUTION: A virtual arc 7 shown by a polygonal line connecting a node n1 with a node n2 is supposed the identification (ID) names of both the nodes n1, n2, the positional information (Ax, Ay) of the start point A of the arc 7 and the positional information (Bx, By) of the end point B of the arc 7 are inputted to a network graphic editing means 1. The means 1 outputs the positional information (X1, Y1), (X2, Y2) of the node n1 and the positional information (X3, Y3), (X4, Y4) of the node n2 and the positional information (Ax, Ay) of the start point A of the arc 7 and the positional information (Bx, By) of the end point B based upon the inputted positional information. An arc deciding means 2 decides which side includes the intersecting point S between the arc 7 and the node n1. The means 2 finds out the coordinates (Sx, Sy) of the intersecting point S, and the coordinates (Ex, Ey) of the intersecting point E between the arc 7 and the node n2 and the finds of the positional information (P1x, P1y) of a folded point P1.