A near-field light detecting apparatus providing a high spatial resolution comprises a near-field light sensor for converting near-field light from the surface of a sample into an electric signal, and a voltage source for applying a predetermined voltage to the near-field light sensor through wires. The near-field light sensor comprises a nanotube which has an insulating property in a predetermined area. Electronic excitation is induced by the near-field light in two areas separated by the insulating area to convert the near-field light into the electric signal.