PURPOSE: To vary the upper limit and lower limit of a visual field and to prevent marks other than a reference mark from being detected erroneously by providing a mask area in the visual field of a visual means and masking scan data.

CONSTITUTION: The visual means (camera) 12 is attached to the pedestal 11 of a press machine 10. This camera 12 is incorporated in a line image sensor arranged in an Y-axial direction. A feeder device 20 has a clamper carrier 25 which clamps and moves a plate-like material 21 in an X-axial direction and rails which guide tables 26 and 27 for supporting the material 21 in the Y-axial direction. Firstly, the material 21 is positioned roughly within the range of a visual field V' obtained by reducing the upper and lower limits of the visual field V on the basis of X and Y position coordinate data on the position of the feeder device 20 and also within the range from a subscan starting position to an ending position. Then, a sensor control part and an X-axis driving mechanism are put in operation to measure the center position MC of a mark M, and high- precision positioning is carried out.