PURPOSE: To prevent the arrow mask detection leakage in the arrow mark graphic recognization by the graphic recognizing processing by detecting the arrow mask graphic by the vector direction of three segments to constitute the arrow mark graphic and the existing number of the arrow mask deciding segment.

CONSTITUTION: In detecting processings 1 and 2 of the vane candidate line of an arrow mark, first, a segment LN 1 having the edge surface is detected by the processing 1, and continuously, a segment LN 2 connected to the segment LN 1 detected by the processing of the processing 1 is detected by the processing 2. Next, an LN 2 edge point detecting processing 3 decides whether or not the segment LN 2 detected by the processing 2 has an edge point. When the processing 3 has the edge point, a segment length deciding processing 4 of the LN 2 compares the segment length of segments LN 1 and LN 2 detected by the processing of the processings 1 and 2, the difference between segment lengths is smaller than the prescribed threshold, and then, the segments LN 1 and LN 2 are made into a vane candidate segment, and the arrow mark constituting deciding segment is detected 6, and an arrow mark constituting deciding segment LN 3 connected to the segments LN 1 and LN 2 is detected.