To solve the problem that, for the purpose of preventing the cutting of a d.c. arc or an a.c. arc, two kinds of d.c. power circuits with different voltages are required, thus a circuit constitution is made complicated.

In an arc welding machine where a d.c. conversion circuit is formed by a change-over-switch in d.c. and an a.c. conversion circuit is formed in an a.c., so as to output the d.c.-a.c., the arc welding machine is provided with: a superpose-reignition power circuit where a rectification circuit is connected to the second winding in the main transformer, superpose voltage is generated by the first smoothing circuit in d.c., and reignition voltage is generated by the second smoothing circuit in a.c.; and a superpose-reignition control circuit where superpose voltage is applied to a space between an electrode and a workpiece in a d.c. mode, and when the electrode is changed from the minus state to the plus state in a.c., reignition voltage with a square wave shape in which peak value steeply changes is superposed on the secondary side voltage of the main transformer, and is applied to a space between the electrode and the workpiece.