According to the present invention, an adapter kit is provided in which a welding torch having a single nozzle structure is switched to a welding torch having a double nozzle structure. In an adapter kit (20A) which is installed in a switchable manner in a welding torch (1A) having a single nozzle structure, the present invention includes an attachment (7A) which is mounted to the torch body (5) in a state where the torch nozzle (inner nozzle) (6A) is inserted into the attachment after the gasket is removed and in which a flow channel that supplies a second shielding gas is provided, and an outer nozzle (8A) that is mounted to the attachment (7A) surrounding the periphery of the torch nozzle (6A) and discharges a second shielding gas.