MicroRNAs (miRNAs) are potentially useful biomarkers for the evaluation of cancers and chemical toxicity. However, simple and easily operated detection techniques are increasingly needed to enable their use in the biomedical and environmental fields. In this study, we investigated sequence-specific miRNA detections by surface plasmon resonance (SPR) and electrochemical (EC) imaging measurements on SPR-EC chips possessing a Au electrode array modified with probe DNAs and by employing a miRNA-detection-selective signal amplification method. MiRNAs were sequence-specifically detected by SPR and EC imaging measurements on a single chip. SPR signals were enhanced by a combination process employing the sequence-specific hybridization of miRNAs with the probe DNA, the extension reaction of polyadenine [poly(A)] tails on the 3’ end of the miRNAs by poly(A) polymerase, the binding of T30-Au nanoparticles (AuNPs) to the poly(A) tails, and the reduction reaction of Ag+ catalyzed on the AuNPs, depositing a gray precipitate on the surface. The oxidation reaction of the accumulated Ag precipitation additionally provides EC signal amplification. This process considerably and sequence-specifically amplifies the SPR and EC signals. This setup constitutes a simple and feasible signal amplification method for the on-site detection of miRNA biomarkers.