Single-walled carbon nanotubes (SWNT) should be among the best candidates for ultra-small functional units in ultra-sensitive, low-energy consumption nano-devices. Several characteristics of the electronic band structure of individual SWNT have been obtained successfully employing recently developed experimental techniques in confocal micro Raman spectroscopy, scanning tunneling microscopy and fluorescence methodology. In addition to band profiles, information regarding absolute potential of energy levels of the states is known to be essential to nano-device fabrication. Several estimations of energy levels from measurements of work function were attempted in vacuums and in electrolyte solution; however, all of these measurements utilized bulk bundle SWNT, which display certain distributions with respect to size. The diameter-dependent absolute potential of individual tubes has never been observed experimentally. The present report reveals that the energy position of the Fermi level of individual SWNT relative to the vacuum level depends on tube diameter based on measurements of resonance Raman intensity of individual SWNT under electrochemical potential control. Absolute potential of the Fermi level was found to be extremely sensitive to tube diameter. Structural dependence of metallic tubes is larger than that of semiconducting tubes. The values obtained can provide absolute potential maps of single SWNT at interface.