A
voltage is applied across a counter
electrode and a
working electrode while an oxidant of a
redox substance is in contact with the counter
electrode and is not substantially in contact with the
working electrode, and thereby
metal that readily undergoes electrolytic oxidation forming at least part of a surface of the
working electrode is oxidized while the oxidant in contact with the working
electrode is reduced, such that a current produced upon the oxidation and the reduction is measured. According to the above constitution, while lowering the
voltage applied across the working electrode and the counter electrode, a
hematocrit value of a blood sample can be measured stably with satisfactory detection sensitivity. This measurement can be carried out, for example, with a sensor
chip comprising a working electrode 11, a counter electrode 12, and a blood sample holder 14 in communication with a blood sample inlet 16, wherein a portion 31 of the working electrode 11 is disposed closer to the blood sample inlet 16 than a portion 32 of the counter electrode is, a surface of the portion 31 includes the
metal that readily undergoes electrolytic oxidation, and a
reagent 13 containing the oxidant is disposed in contact with the portion 32.