In this study, we developed a surface plasmon resonance (SPR)-based sensing method with high sensitivity for the detection of brain natriuretic peptide (BNP), the level of which is a useful indicator of heart failure. In our system, BNP was detected by a sandwich-type immunoassay using two kinds of monoclonal antibodies: primary and secondary. The SPR signal was amplified by using streptavidin-conjugated nanobeads approximately 50 nm in diameter that specifically accumulated on the secondary antibody via biotin-avidin bonding. By this method, the SPR signals were highly intensified, and thus picogram levels (pg/ml) of BNP could be detected. The streptavidin nanobead amplification method realizes detection of very low levels of BNP, as is necessary for the clinical diagnosis. It is also expected that our streptavidin nanobead amplification method can be applicable to the detection of other hormones and tumor markers that are present in only low concentrations within the human body.