Inhibition effects of (+)-catechin-aldehyde polycondensates against the activity of proteinases, Clostridium histolyticum collagenase (ChC) and human neutrophil elastase (HNE) causing proteolytic degradation of extracellular matrix (ECM), have been investigated. In normal tissues, a balance is reached between the formation and destruction of ECM, leading to a state of homeostasis. However, uncontrolled destruction of ECM contributes to tumor invasion and metastasis. In the measurement of the inhibition activity on ChC and HNE, the polycondensates exhibited superior effects compared to the catechin monomer. Kinetic assays of ChC and HNE inhibition by the polycondensate clearly showed a mixed-type inhibition. These data demonstrate that the polycondensates are a new class of proteinase inhibitors useful for a potent therapeutic agent.