In a biosensor that detects introduction of a sample liquid into a specimen supply path using a detecting electrode, a means of improving accuracy of detection is provided. The biosensor has: an electrode system including measuring electrode, counter electrode, and detecting electrode on first electrically insulating support; specimen supply path for introducing the sample liquid; and reagent layer used for quantifying a substrate contained in the sample liquid. The means is characterized in that detecting electrode is spaced from measuring electrode by a distance sufficient for the sample liquid to sufficiently cover measuring electrode before the sample liquid reaches detecting electrode.