A method of producing a carbon nanostructure is provided that enables production of a high-quality carbon nanostructure with a high yield. The method of producing a carbon nanostructure includes supplying a feedstock gas to a catalyst and growing a carbon nanostructure by chemical vapor deposition. A gas X that is derived from the feedstock gas and that comes into contact with the catalyst contains a hydrocarbon A having at least one cyclopentadiene skeleton and a hydrocarbon B having at least one acetylene skeleton. A total volume concentration [A] of the hydrocarbon A is at least 0.06%.