PURPOSE: To obtain uniform and high welding quality irrespective of a welding object by reading reference data by sampling into a memory bank for arc and a memory bank for short circuiting and controlling the relative position between the welding torch and base materials as well as speed according to said data.

CONSTITUTION: If the characteristics to be used as a standard for welding current Ai and welding voltage Ei are designated as (a), a', the memory address corresponding to the current Ai is determined and the reference welding voltage in the stage of arc corresponding to each address is stored as arc memory data into a prescribed memeory bank. The reference welting voltage in the stage of short circuiting is registered as short circuiting memory data into a prescribed memory bank. The reference welding voltage corresponding to the current Ai is read from the memory in the stage of welding and the deviation from the actual welding voltage is determined. The space between welding torch and base material is controlled in the direction where said deviation is made zero. The speeds of the vertical and horizontal movements of the welding torch are controlled at the same time so that the combined speed is maintained constant.