To early detect a symptom indicating abnormality of equipment by diagnosing the presence/absence of a positional change of an entrance clamp device on its taper wedge and work side and on the drive side.

First, an amount of vertical travel is measured for a taper wedge 17, which is attached to an entrance clamp device 2, by a position detector 19a, converted to an electric signal by a converter 21a and inputted in a computer 23. The abutting quantity set in the computer 23 is compared with the measured quantity, and then an diagnosis is made for the prescribed positioning of the taper wedge 17 so that a prescribed abutting quantity may be attained. With the entrance clamp device 2 moved to an advancing position, its position is detected by position detectors 19b, 19c, converted to a signal and inputted in the computer 23. Change in the advancing position is diagnosed by the computer 23 for the entrance clamp device 2. Thus, the change in the abutting quantity can be diagnosed for a preceding and succeeding strips 12, 13 at the time of welding.