Voltammetric behaviors of binding between soybean agglutinin (SBA) having sites for galactosamine and the sugar were investigated. A self-assembled monolayer (SAM) of galactosamine was designed on an Au electrode surface, and SBA-galactosamine interaction was monitored through ferrocyanide ion as marker. Electrode responses of ferrocyanide ion decreased with increasing the concentrations of the SBA. This is because SBA selectively combines to sugar part on the electrode surface. To confirm the binding, voltammetric behaviors of several lectins not having sites for galactosamine were compared to that of SBA.