PURPOSE: To perform highly accurate detection at a high speed, by optically measuring the distance to a plurality of points on an object material using a time division method, and judging the position deviation of a welding torch and a welding line based on the measured data.

CONSTITUTION: A distance sensor 10 has a function of detecting the distance to a plurality of points on a groove position 11, which are separated by a specified minute distance, from a torch 3 in the progressing direction of welding. The signal obtained by the distance sensor 10 is processed by a judging and processing circuit 13. Thus the amount of position deviation between the torch 3 and a welding line 2 is obtained. The signal is sent to a control circuit 14, and a driving mechanism for the torch 3 is driven so as to correct the position deviation.