To prevent breakage of a weld zone in advance by detecting the deflection of outlet side and inlet side clamp devices in the plate width direction, and outputting an abnormal alarm when the deflection exceeds the preset reference value to rapidly detect generation of the level difference between a butted part of a preceding strip and of a succeeding strip.

When a rear end part of a preceding strip 12 is fixed by upper and lower clamp pads 17, 18 of an outlet side clamp device 3, the deflection in the plate width direction of a rear pad piece 18b is detected by a displacement meter 29a, the detected signal is transmitted to converters 32a-32d through signal lines 31a-31d, the converted signal is transmitted to a relay panel 34 through signal lines 33a-33d, and the signal is transmitted to an operation device 36 through a signal line 35. The difference between the minimum value and the maximum value of the deflection in the plate width direction of a rear pad piece 18b is calculated, the difference is compared with the preset reference value, and an abnormal alarm is displayed on a display device 43 when the difference exceeds the reference value. Similar operation is performed even by an inlet side clamp device 2.