PURPOSE: To detect even a narrow groove at high precision, by eliminating electromagnetic mutual interference of the detecting coil of captioned sensor.

CONSTITUTION: AC power supplies 3, 4 differing in frequency are connected to detection coils 11, 21 of a sensor S which is integral with welding torches set parallel on a plane vertical to, for example, work W face WS and groove line WL. Both the coils 11, 21 change in the value of Q due to the effect of work W, and the balance is broken in both bridges B1, B2, and output signals are issued. These signals are passed through amplifiers 5, 6 and filter circuits 7, 8, and only 1 and 1.3MHz signals are taken out and are inputted into an amplifier 30 through rectifying circuits 9, 10. When distances l1, l2 between groove line WL and coil center line LC, and IC and 2C are not equal to each other respectively, it results in 1P≠2P, and the outputs in the circuits 9, 10 vary, so that the output value of the amplifier 30 is no longer zero. Thus, depending whether the output is positive or negative, the deviation direction with respect to the sensor S and groove line WL is judged.