PURPOSE: To simplify circuit and to improve arc welding by providing freewheeling diodes and a reactor, etc. and performing inversion of current polarity and control of current value only by controlling switching elements.

CONSTITUTION: An arc welding machine consists of a DC power source 6, plural switching elements T1W4, a driving circuit 16 for driving said elements, freewheeling diodes D1W4 and a reactor 11. The elements T1W4 form a bridge circuit 13 and are connected to the power source 6. The diodes D1W4 are provided for each of the elements T1W4. The reactor 11 is connected to the output side of the circuit 13. The circuit is simplified by the above-mentioned welding machine and base metals 5 are satisfactorily welded by a torch 3.