PURPOSE: To decrease the step in a weld zone and to prevent the rupture accident during rolling owing to the step in butt welding of traveling plate materials by moving vertically an anvil for clamping the succeeding material by as much as half the difference between the thickness at the rear end of the preceding material and the thickness at the preceding end of the succeeding material.

CONSTITUTION: The signal for the thickness detected with a device 13 for detecting plate thickness when the rear end of a preceding material S1 is grasped under pressure by a clamp 11 on an outlet side and the signal for the thickness detected with a device 19 for detecting plate thickness when the front end of a succeeding material S2 is grasped under pressure by a clamp 17 on an inlet side are put into a microcomputer 23 into which coil information 30 is preliminarily put. The judgement for changing the thickness above or below the half value of the difference in thickness is put into a signal converting panel 25 to actuate a mechanism 24 for adjusting the height of electrodes by a cylinder 27 so that the height of an anvil is adjusted by an instructed size 28. Electrodes 15, 21 are clamped by a welder control panel 29 after the adjustment, then welding is performed. The step between the butt surfaces is thus minimized and the rupture by the step during rolling is prevented.