Ferrocenylnaphthalene diimide (FND) having double-stranded DNA preference was used as an electrochemical hybridization indicator and applied to the electrochemical hybridization assay (EHA) coupled with a DNA probe-immobilized gold electrode. The effect of DNA probe length and the position of SNP (single nucleotide polymorphism) site on the DNA probe were studied in the SNP analysis concerned with the side effect of an anticancer drug, irinotecan. SNP analysis was achieved with high precision when a DNA probe carrying the SNP site near the terminus of a DNA probe was used in the EHA.