We have found a method for a diameter-selective removal of metallic single-walled carbon nanotubes (m-SWCNTs) from semiconducting (s-) ones by stirring or sonicating SWCNT powder in tetramethylene sulfone (TMS)/chloroform solution with nitronium hexafluoroantimonate (NO2SbF6: NHFA) and nitronium tetrafluoroborate (NO2BF4: NTFB). Positively charged nitronium ions (NO2+) were intercalated into nanotube bundles, where the intercalation was promoted also by the counterions. Nitronium ions selectively attacked the sidewall of the m-SWCNTs due to the abundant presence of electron density at the Fermi level, thus yielding stronger binding energy as compared to the counterpart s-SWCNTs. The s-SWCNTs were left on the filter after filtration, whereas the m-SWCNTs were disintegrated and drained away as amorphous carbons. The effectiveness of removing m-SWCNTs was confirmed by the resonant Raman spectra and absorption spectra.