To measure a glucose level in blood, a method which utilizes a disposable biosensor is often employed as an easy method of measurement (See Patent Document 1, for example). Like the glucose sensor 9 shown in FIGS. 9 and 10 of the present application, there exists a glucose sensor configured to measure the response current necessary for the computation of a blood glucose level by using a working electrode 90 and a counter electrode 91. In the glucose sensor 9, blood is moved by a capillary force generated in the capillary 92, and the amount of electron transfer upon the reaction between blood and a reagent is measured as the response current. As shown in FIGS. 10 and 11, the reagent is retained as a reagent portion 95 in an opening 94 of an insulating film 93 on a substrate 96. The reagent portion 95 is formed to be highly soluble so that the reagent is dispersed in blood when blood is introduced. As shown in FIGS. 9 and 10, the capillary 92 is defined by stacking a cover 98 to the substrate 96 via a spacer 97 formed with a slit 97a. 
The speed of movement of blood (suction force acting on blood) in the capillary 98 depends on the wettability of a surface of the cover 98 and the solubility of the reagent portion 95. Generally, the wettability of the cover 98 and the solubility of the reagent portion 95 deteriorate with time or depending on the temperature. As better shown in FIG. 10, on the surface of the substrate 96, a stepped portion 99 is formed due to the provision of the opening 94 in the insulating film 93. Therefore, as shown in FIGS. 12A and 12B, the blood B introduced into the capillary 92 may be stopped at the stepped portion 99 in moving the capillary 92. Such a phenomenon is more likely to occur as the suction force in the capillary 92 deteriorates, i.e., as the wettability of the cover 98 and the solubility of the reagent portion 95 deteriorate.
The blood B which has stopped moving at the stepped portion 99 may stop its progress in that state. In some cases, however, the blood B moves again gradually and then suddenly moves largely. When the blood B moves again, the amount (concentration) of electron mediator existing around the working electrode 90 and the counter electrode 91 suddenly changes. In such a case, as indicated by phantom lines in FIG. 13, the measured response current suddenly increases. The phenomenon that the blood B moves again does not necessarily occur at each time of blood glucose level measurement, and the timing at which the blood movement phenomenon occurs is not constant in glucose sensors 9. Therefore, the glucose sensor 9 in which the blood B may move again has poor reproducibility of current measurements, and hence has poor reproducibility of blood glucose levels obtained by computation.
Patent Document 1: JP-A 8-10208