PURPOSE: To shorten the tact time of an arc welding robot or an automatic machine by setting the lag time long when a welding command signal is in an ON state and shortening the lag time sufficiently when the signal is in an OFF state.

CONSTITUTION: A DC current is outputted to a consumable electrode 3 from an AC power source 1 and a DC power source 2 to weld materials 6 to be welded. The arc welding machine is provided with a welding current detection circuit 7 and a timing circuit 10 which immediately operates by output of this DC current, starts timing by output annihilation of the welding current detection circuit and returns after the set time. When the welding command signal is inputted and the welding command signal 11 exists by using a lag time changeover circuit, the return time of the timing circuit 10 is set long and when the welding command signal 11 does not exist, the return time of the timing circuit 10 is set short. An output signal of the timing circuit 10 is then outputted as a welding current energizing stopping signal. Consequently, the energizing stop of a welding current at a welding finishing part can be detected early.