An active portion of a nano-size element is selectively destroyed by applying an electric potential to the nano-size element having a carbon network structure of carbon as an outer shell while immersed in an electrolytic solution. For example, an electric potential of 1.20 to 1.60 V (RHE) is applied to the carbon nanotubes in electrolytic solution so as to oxidize the active portion in each carbon nanotube and to thereby selectively remove the ends. Consequently, an open end is created in the carbon nanotube.