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).