PURPOSE: To provide an appropriate and stable track of a welding robot in a tack welded part.

CONSTITUTION: The sensing beam S of a laser beam sensor 4 which is fixed to a fixing part 2 of a robot arm 1 and supported together with a welding torch 3 scans the range which is preceded by the distance D in the robot advancing direction from the tool point T. The teaching points Pn, Pn+1 are set at the position across the tack welded part F. The relationship d<L<D is established, where the dimension of the tack welded part is (d), the distance between the teaching points Pn and Pn+1 is L. For the interval between the teaching points where no tack welded parts are present, the normal welding tracking is executed, while for the part corresponding to the interval PnPn+1, the robot track is determined by making use of the whole or a part of the robot positional data on the points 0Qn-6Qn in the vicinity of each end side of the interval and the sensing output data on the points 0Qn+1-6Qn+1 in place of the sensing data of this interval. When the sensing point Qn+1 corresponding to the teaching point Pn+1 is reached, the robot control by the normal welding line tracking method is re-started.