A simple-structured biosensor is provided for measuring a substrate in a sample rapidly and highly precisely, without significant influence of a peptide included in the sample. A biosensor for measuring the substrate contained in a sample solution includes one or a plurality of insulating base plate(s) 1; a pair of electrodes including a working electrode 2 and a counter electrode 3 provided on the base plate 1; and a reagent 11 for measurement including a redox enzyme and an electron mediator. The sample solution contains a peptide, and the working electrode 2 includes a metal, and at least a part of a surface of the working electrode 2 is covered with a film 10 of an organic compound containing at least one sulfur atom.