Selective ion currents in aqueous calcium chloride and sodium chloride solutions with concentrations of up to 1.0 M were observed with sub-micropipettes in which poly (vinyl chloride)(PVC) films containing ionophores selectively filtered cations. Calcium bis [4-(1, 1, 3, 3-tetramethylbutyl) phenyl] phosphate (HDOPP-Ca) and bis [(12-crown-4) methyl]-2-dodecyl-2-methylmalonate [bis (12-crown-4)] were used as the ionophores to filter calcium and sodium ions, respectively. The selective ion current was observed using a low-current detection system developed from scanning tunneling microscopy. The approximate linear relationship between the ion concentration and ion current suggests that the sub-micropipette probe can be used to detect the intracellular local concentration of a specific ion up to 1.0 M.