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Self-propagating autocatalytic reactions of proteases that can provide high signal amplification have not been applied to affinity-based biosensors owing to the limited number of fast autocatalytic proteolytic reactions available and the self-activation of protease proenzymes. Here, we report that a self-propagating autocatalytic reaction based on the autocatalytic activation of the trypsinogen mutant by trypsin facilitates high signal amplification and a low background level, resulting in a low detection limit for prostate-specific antigen (PSA). A commercially available trypsinogen mutant minimizes the self-activation of trypsinogen by trypsinogen. Trypsin, which is used as a catalytic label in a sandwich-type immunosensor, converts the trypsinogen mutant into trypsin; the generated trypsin then further converts the trypsinogen mutant into trypsin. The autocatalytically produced trypsin proteolytically cleaves the peptide bond of a trypsin substrate, resulting in the liberation of electrochemically active 4-aminophenol (AP). The electrochemical oxidation of AP at a modified indium tin oxide (ITO) electrode induces electrochemical-chemical redox cycling involving the ITO electrode, AP, and a reductant. The triple combination of autocatalytic activation, proteolytic cleavage, and redox cycling results in a high electrochemical signal level. The detection limit for PSA obtained using a trypsin label and trypsinogen (∼7 pg/mL) is lower than that obtained using a trypsin label alone (∼100 pg/mL). This study demonstrated that autocatalytically activating a proenzyme is a very useful method for highly amplifying signals.