The present invention is directed to a biosensor being provided with a suction cavity which sucks a certain amount of sample by means of capillary phenomenon, the biosensor including a flow channel to connect the suction cavity and an analytical cavity furnished with a reagent, wherein, the flow channel has a narrowed section provided with a gap formed by narrowing a flow passage area. The narrowed section has a function to retain the sample in the suction cavity when the sample is sucked therein, and a function to circulate the sample retained in the suction cavity into the analytical cavity through the gap, when a centrifugal force is applied from the outside. The two functions held by the narrowed section enables a simple collection of blood sample, and further enables a simple transfer of the blood sample to the analytical area by applying the centrifugal force to the blood sample, as well as facilitating a component extraction.