Devices, systems and methods for sequencing protein samples are provided. In some examples, currents generated when a monomer passes through between electrodes of a nanogap electrode pair are measured for each of several different distances, so that monomers are identified when compared to a reference physical quantity of a known monomer, which may be obtained from a current measured with a similar inter-electrode distance(s) at which each of plural kinds of monomers are identifiable and ordered with predetermined accuracy and based on a detected physical quantity obtained from a tunneling current, which may be further normalized by the use of one or more reference substances.