To dispense with a safety device for avoiding a transition from a looper-threading state to a stitch-forming state during a gas supply operation of a gas supply source, and to readily thread the looper with one touch, simplified in terms of mechanism and readily performed with a small number of one-handed operations in terms of operation. A meshing clutch (60) for transmitting drive force from a sewing machine motor (M) to a gas supply source (40) during looper threading and to a drive shaft (5) for driving a looper-containing stitch-forming device during stitch formation, is allowed to freely come into contact with or separate from a gas feed drive unit (61) for transmitting drive force to the gas supply source or a stitch-forming drive unit (64) secured to one end of the drive shaft and used for transmitting drive force to the stitch-forming device in accordance with a manual operation of a manual operation unit for switching between looper-threading and stitch-formation, and is configured so that drive force is transmitted from the sewing machine motor via a hollow clutch shaft (22) and so that a connected state is maintained when connected to the gas feed drive unit.