Voltage is applied across a counter
electrode and a
working electrode, with which a blood sample is in contact, in such a state that an oxidant in a
redox substance is not substantially in contact with a
working electrode but is in contact with a counter
electrode and a reductant is not substantially in contact with the counter
electrode but is in contact with the
working electrode, whereby the reductant and the oxidant are respectively oxidized and reduced to measure current produced upon the oxidation and reduction. According to the above constitution, while lowering the
voltage applied across the working electrode and the counter electrode, the Hct value of the blood sample can be measured stably with a satisfactory detection sensitivity. This measurement can be carried out with a sensor
chip comprising a working electrode (11), a counter electrode (12), and a blood sample holding part (14) having
branch parts (18a, 18b). A first
reagent (13) consisting essentially of an oxidant in a
redox substance is disposed in contact with the surface of a counter electrode which faces the
branch part (18b), and a second
reagent (19) consisting essentially of a reductant is disposed in contact with the surface of a working electrode that faces the
branch part (18a).