Abstract: There is provided a concentration sensor for measurement of concentrations of one or plural target substances in a system, with a detection electrode of conductive diamond. The measuring method with the concentration sensor involves measuring the reaction potentials of plural target substances in a sample containing and determining the concentrations of the plural substances by use of the difference between the resulting reaction potentials of the respective substances. The measuring method can also determine the concentration of a single substance in a sample containing one kind of target substance.