To improve start-up ability and welding quality by changing an slope inclination corresponding to a welding output condition and a wire extension at welding start.

A slope setting circuit part 9, as an input of a calculated result of a wire extension calculation part 8 and an output setting instruction value of an output setting instructing part 10, outputs a welding output instruction value in which a wire feed quantity and an output voltage from welding start up to an output setting current are set to a slope shape corresponding to the calculated result of the wire extension calculating part 8, the start condition optimum to the wire extension is outputted.