PURPOSE: To ease the impact force applied on a tool holding arm at work time, by making an electromagnet homopolar with a meeting permanent magnet, floating magnetically the permanent magnet by the upper and lower repulsive force thereof and elastically locking the tool holding arm linked thereto.

CONSTITUTION: Electromagnets 3 and 4 are made homopolar with a meeting permanent magnet 5 and electrification is performed so that the repulsive forces of the electromagnet 4 and permanent magnet 5 lower end become equal to the sum of the weights of a tool holding arm and the member linked thereto and the repulsive forces of the electromagnet 3 and permanent magnet 5 upper end. Consequently, the permanent magnet 5 is magnetically floated, a holding arm 7 is locked elastically as the result and the impact force exerted from a welding torch can be eased.