To more precisely perform arc length control of a MIG arc in a plasma MIG welding method for simultaneously generating a MIG arc and a plasma arc from one welding torch.

A MIG welding voltage limit value Vf is calculated by limiting a MIG welding voltage Vwm into a variation range ±ΔVc with the detected value of MIG welding voltage Vwm as an input and with a reference voltage waveform Vc as the central voltage value, and a pulse period Tf or a peak period Tp is changed on the basis of the MIG welding voltage limit value Vft, and the arc length control for the MIG arc is performed. The above variation range ΔVc is changed in accordance with the value Iwpr of the plasma welding current. In this way, as the value of the plasma welding current increases, power that the plasma arc constrains the MIG arc is increased, thus the variation range ΔVc is increased, the superposing of the abnormal voltage is suppressed, and also, voltage variation accompanying the variation of the arc length can be correctly detected.