PURPOSE: To rapidly detect the existence of an intersecting point of two straight lines without using division by detecting the existence of the intersecting point of the two straight lines at the time of deciding that the coordinates of both the ends of one straight line are positioned on respectively opposite sides with the other straight line as a border line.

CONSTITUTION: The 1st arithmetic part 3 in a criterion computing means 1 computes the positions of both the end points (c), (d) of the other straight line B respect to one straight line A connecting points (a), (b) and decides whether the points (c), (d) are arranged on respectively opposite side positions with the straight line A as a border line or not. Similarly, the 2nd arithmetic part 4 decides the positions of the points (a), (b) with the straight line B as the border line. When respective points are decided to be positioned on the opposite sides by the arithmetic parts 3, 4, a deciding means 2 decides the existence of the intersecting point of the two straight lines A, B without executing division, so that the existence of the intersecting points of two straight lines can rapidly be detected.