The biosensor of the present invention has a reaction layer that contains at least an oxidoreductase and an electron acceptor and is formed on an electrode system including a working electrode and a counter electrode on a base plate. The working electrode, the counter electrode, and lead conductors connecting with these electrodes are made of carbon. The counter electrode is formed in a substantially C shape, and the working electrode is arranged in a recess of the substantially C-shaped counter electrode. An insulating layer surrounds the counter electrode and the working electrode. A cover member with a groove is combined with the base plate to define a sample supply channel that is formed therebetween and runs toward the electrode system. The groove extends from one end of the base plate and passes over the electrode system, and the insulating layer extends to an air hole formed in a terminal of the groove of the cover member. This arrangement prevents exposure of part of the lead conductor connecting with the working electrode to the sample solution and development of an error in measurement.