The N-2 adsorption on the internal and external surfaces of single-walled carbon nanotubes of the continuous model at 77 K was compared with that on the structureless graphite surface with grand canonical Monte Carlo (GCMC) simulation. The inapplicability of the BET analysis for evaluation of the surface area of the internal and external surfaces of the nanombe was explicitly shown and the reason was explained with the monolayer structure difference. The ordinary alpha(s)-plot analysis using the standard isotherm of the flat graphite surface did not provide a reasonable method for the internal surface area. However, if we adopt the adsorption isotherm of the internal surface of the pore width being large enough to remove the enhanced potential effect, the subtracting pore effect method (SPE-IC method) using its alpha(s)-plot gave an accurate surface area for the internal surface except for the pore width = 2 nm. The cause for the inapplicability of BET analysis was discussed with the radial distribution function of the monolayer.