A welding robot has a gun main body, a stationary electrode tip which is immovable relative to the gun main body, and a movable electrode tip which is mounted on the gun main body and is opened and closed by a pressing source. The gun main body is movable by a driving source in a direction in which both the electrode tips lie opposite to each other. In teaching the welding robot by the movement thereof to each weld spot position of a workpiece, the movable electrode tip is moved in a closing direction relative to the gun main body by the pressing source in a state in which the gun main body is kept stationary at a predetermined position relative to the gun support bracket. The stationary electrode tip is moved in the closing direction by a relative movement of the gun main body relative to the movable electrode tip after the movable electrode tip has come into contact with the workpiece. A position at which the stationary electrode tip contacted the workpiece is detected. This position is stored as teaching data.