A highly reliable biosensor is provided. The biosensor of the present invention includes two or more electrodes (2a,2b,2c), and a support (3) supporting the two or more electrodes (2a,2b,2c). One or more of the electrodes (2a,2b) each include a substrate (21) having a metal surface, and a protective layer (22) covering at least part of the surface. The protective layer (22) is a continuous film made of carbon and having a thickness of at least 1 µm.
