The present invention provides a biosensor for detecting the interaction of substances on a substrate based on surface plasmon resonance, the substrate having a metal film including approximately 90% by mol or more and less than approximately 99.95% by mol of Ag as a first metal element, and approximately 0.05% by mol or more and less than approximately 10% by mol of at lease one of Au, Pt, Cu, Bi, Nd, Ti or Sb as a second metal element.