The present invention relates to a biosensor, and it is an object thereof to suppress degradation by light. A biosensor (2) comprises a blood component measurement working electrode (5), a blood component measurement counter electrode (6), and a reagent component (9). The reagent component (9) is provided near the blood component measurement working electrode (5) and the blood component measurement counter electrode (6). The reagent component (9) includes a mediator, an oxidoreductase, and a substance that absorbs light with a wavelength of 400 to 500 nm. An insulated substrate (4) is provided, on the surface of which is disposed the blood component measurement working electrode (5) and/or the blood component measurement counter electrode (6). The reagent component (9) is disposed on the one or more electrodes on the insulated substrate (4).