Disclosed is a biosensor capable of measuring a total concentration of plural types of amino acids. An amino-acid biosensor (200) for measuring a total concentration of a plurality of specific amino acids, comprises a measuring electrode (202) which includes as components a mediator and an enzyme which selectively act on the plurality of specific amino acids each serving as a substrate, and a counter electrode (203). The enzyme has a substrate affinity to each of the plurality of specific amino acids. The enzyme is operable to catalyze a reaction in each of the plurality of specific amino acids as a substrate so as to form a reaction product, and the mediator is operable, during amino-acid concentration measurement, to carry electrons between the reaction product and the measuring electrode. The amino-acid biosensor is designed to apply a voltage between the measuring electrode and the counter electrode during the measurement in such a manner that, in an analytical curve representing a relationship between an applied voltage and a current value for each of the plurality of specific amino acids, the applied voltage includes a voltage allowing the variety of the current values for the amino acids at the same applied voltage to fall within a given range.