A welding method, comprising: an inert gas supply step to supply inert gas to a surface of a metallic material from the inside of a cylinder welding nozzle; a heating step to heat the surface of the metallic material where the inert gas has been supplied by the welding nozzle in the inert gas supply step; and an oxygen introduction step to introduce oxygen in atmosphere that is suctioned due to a reduction of pressure generated in association with a flow of the inert gas in the inert gas supply step to a molten pool generated on the surface of the metallic material in the heating step characterised in that: the welding nozzle comprises: a nozzle inner cylinder through which the inert gas is distributed, and a nozzle outer cylinder through which the atmosphere that is suctioned is distributed through a gap (g) with the nozzle inner cylinder while surrounding a side of the nozzle inner cylinder; in the inert gas supply step, the inert gas is supplied to the metallic material from the inside of the nozzle inner cylinder; in the oxygen introduction step, while the atmosphere that is suctioned is distributed through the gap between the nozzle inner cylinder and the nozzle outer cylinder, oxygen in the atmosphere is introduced into the molten pool; the welding nozzle further comprises a gap variable unit that can adjust size of the gap between the nozzle inner cylinder and the nozzle outer cylinder; and in the oxygen introduction step, an amount of atmosphere that is distributed through the gap between the nozzle inner cylinder and the nozzle outer cylinder is controlled by adjusting the size of the gap between the nozzle inner cylinder and the nozzle outer cylinder with the gap variable unit, and the amount of oxygen introduced into the molten pool is thereby controlled. an inert gas supply step to supply inert gas to a surface of a metallic material from the inside of a cylinder welding nozzle; a heating step to heat the surface of the metallic material where the inert gas has been supplied by the welding nozzle in the inert gas supply step; and an oxygen introduction step to introduce oxygen in atmosphere that is suctioned due to a reduction of pressure generated in association with a flow of the inert gas in the inert gas supply step to a molten pool generated on the surface of the metallic material in the heating step characterised in that: the welding nozzle comprises: a nozzle inner cylinder through which the inert gas is distributed, and a nozzle outer cylinder through which the atmosphere that is suctioned is distributed through a gap (g) with the nozzle inner cylinder while surrounding a side of the nozzle inner cylinder; in the inert gas supply step, the inert gas is supplied to the metallic material from the inside of the nozzle inner cylinder; in the oxygen introduction step, while the atmosphere that is suctioned is distributed through the gap between the nozzle inner cylinder and the nozzle outer cylinder, oxygen in the atmosphere is introduced into the molten pool; the welding nozzle further comprises a gap variable unit that can adjust size of the gap between the nozzle inner cylinder and the nozzle outer cylinder; and in the oxygen introduction step, an amount of atmosphere that is distributed through the gap between the nozzle inner cylinder and the nozzle outer cylinder is controlled by adjusting the size of the gap between the nozzle inner cylinder and the nozzle outer cylinder with the gap variable unit, and the amount of oxygen introduced into the molten pool is thereby controlled. the welding nozzle comprises: a nozzle inner cylinder through which the inert gas is distributed, and a nozzle outer cylinder through which the atmosphere that is suctioned is distributed through a gap (g) with the nozzle inner cylinder while surrounding a side of the nozzle inner cylinder; a nozzle inner cylinder through which the inert gas is distributed, and a nozzle outer cylinder through which the atmosphere that is suctioned is distributed through a gap (g) with the nozzle inner cylinder while surrounding a side of the nozzle inner cylinder; in the inert gas supply step, the inert gas is supplied to the metallic material from the inside of the nozzle inner cylinder; in the oxygen introduction step, while the atmosphere that is suctioned is distributed through the gap between the nozzle inner cylinder and the nozzle outer cylinder, oxygen in the atmosphere is introduced into the molten pool; the welding nozzle further comprises a gap variable unit that can adjust size of the gap between the nozzle inner cylinder and the nozzle outer cylinder; and in the oxygen introduction step, an amount of atmosphere that is distributed through the gap between the nozzle inner cylinder and the nozzle outer cylinder is controlled by adjusting the size of the gap between the nozzle inner cylinder and the nozzle outer cylinder with the gap variable unit, and the amount of oxygen introduced into the molten pool is thereby controlled.