PURPOSE: To make weld line tracking control possible when welding for rotating/traveling work is executed under the state that the torch supported by a robot is fixed.

CONSTITUTION: A cylindrical work W is rotated in the arrow direction mark as an additional axis. A welding torch 2, laser beam sensor 3 are supported at an arm tip part A. A scanning laser beam LB of a laser sensor 3 is projected to the front region of a weld point 4, a beam spot locus 5, is formed in the direction perpendicular to a weld line 6. By photographing with a camera C, a weld line position 7 is detected. At welding of a first layer, tracking control based on the data for position correcting prepared from sensor data is executed, along with this action, data for correcting at every fixed quantity traveling of additional axis is written in the prescribed region of buffer memory. At welding of a second layer and after, the written in data for position correcting is read out at every fixed quantity traveling of additional axis and is used for tracking travel control.