To grasp a dynamic characteristic of a welding power source in response to the variation of an arc current and a wire feeding speed or the like by inputting a difference between output signals of an output voltage detector and output signals of an arc voltage setting signal arithmetic circuit and connecting an analog element in parallel to a short circuit generating circuit wherein a conducting quantity varies in proportion to the input.

A conducting quantity of a transistor 23 varies until output signals Vr of an arc voltage setting signal arithmetic circuit 16 become equal to output signals Vf of an output voltage detector 20. Therefore, when output signals Vr of the arc voltage setting signal arithmetic circuit 16 rise, differential signals ΔV=Vf-Vr become small, a base current of a transistor 23 becomes small and a conducting quantity of the transistor 23 is also reduced. A voltage between output terminals Tm1 and Tm2 rises and an arc voltage rises based on an output voltage and current characteristic of a DC arc welding power source 1. To the contrary, when output signals Vr of the arc voltage setting signal arithmetic circuit 16 drop, the arc voltage drops.