A method for producing a carbon nanostructure according to an aspect of the present invention is a method in which a carbon nanostructure is produced between a base body and a separable body while the separable body is relatively moved away from the base body, the base body including a carburizable metal that is a principal constituent, the separable body including a carburizable metal that is a principal constituent, the separable body being joined to or in contact with the base body in a linear or strip-like shape. The method includes a carburizing gas feed step, an oxidizing gas feed step, a heating step in which the portion of the base body at which the base body and the separable body are joined to or in contact with each other is heated, and a separation step in which the separable body is relatively moved away from the base body.