PURPOSE: To automatically detect the standard pattern which is formed on a workpiece and improve precision by detecting the distance to the surface of a workpiece by the scan by a sensor in the perpendicular direction for the extension direction of the standard pattern and emphasizing the standard pattern corresponding position through the prescribed folding calculation processing for the detection signal.

CONSTITUTION: The distance to a workpiece, i.e., the surface of a construction material board 10 is detected by a distance sensor 11. This detection is carried out by scanning the position sensor 11 in the perpendicular direction for the extension direction of the standard pattern by a scanning mechanism 12. The detection signal is inputted into a calculation control part 13. The calculation control part 13 performs sampling in a constant cycle for the detection signals and adds the values, and obtains the average value. Further, the previously determined folding calculation is carried out by a filter having the impulse response which is obtained by removing the dc component from the wave form of the average value, and the standard pattern is judged. The calculation control part 13 receives the set value from a setting part 14, and controls a shift mechanism on the basis of the detected standard pattern position.