DNA chips have been much interested in the detection and the analysis of genes because the sequence of human genome has been determined by the worldwide human genome project. GeneChip (Affimetrix Co.) is the most popular DNA chip, but is very expensive and needs a huge fluorescence detection equipment. We have been developing an electrochemical gene detection method, named as DNA sensor, in order to supply an easy and cheap device of the gene detection. In recent years, we have focused on the application of the DNA sensor to DNA chip aiming at the bed-side diagnosis of genes. We introduce here the outline of the DNA sensor and the electrochemical DNA chip for the assessment of interferon therapy in hepatitis C patients.