PURPOSE: To obtain similar accuracy with that of a robot as an NC machine by inputting and storing a local picture near the starting point and finishing point of welding from an image sensor when learning simultaneously with NC data, and comparing the picture inputted under the same condition with the time of learning and the local picture when playing back.

CONSTITUTION: A picture inputted from an image sensor 11 is edge detected in an edge detecting section 12, and converted to a contour line picture. Correlation coefficient between a local picture stored in a local picture memory 15 and the new contour line picture converted in the edge detecting section 12 is calculated in a correlation coefficient calculating section 16, and the result is sent to a peak value address detecting section 17. The peak value address detecting section 17 detects coordinate in the contour line picture that makes correlation coefficient maximum, and sends to an address comparator section 19. The address comparator section 19 compares coordinate obtained by the peak value address detecting section 17 and coordinate stored in a reference position storing section 18, and detects basing on the result.