What provided is a biosensor including: a substrate having a surface with first and second regions adjacent each other; a magnetoresistance effect element disposed on the first region; a soft magnetic thin film on the second region; a protective film disposed on both the first region and the second region, covering a surface of the soft magnetic thin film, and disposed on the top part of the second region and contains an affinity substance recognizing the biomolecule on the outer surface of the second region exclusively; and an MR cover film disposed on at least the top part of the first region, disposed on the protective film above the magnetoresistance effect element, and being free of the affinity substance, wherein the soft magnetic thin film transmits an in-plane component of a stray magnetic field of magnetic beads to the magnetoresistance effect element.