A flow cell device is incorporated in the sensor unit for surface plasmon resonance (SPR) assay. A box shaped flow cell body has a lower surface, and is disposed in contact of the lower surface with a sensing surface for detecting reaction of sample. Two flow channels are formed in the flow cell body, for flow of the sample. Each of the flow channels include a fluid passageway, formed in the lower surface, for flow of the sample in contact with the sensing surface. Two end cavities are formed in an upper surface of the flow cell body and open therein, to extend from ends of the fluid passageway. The flow channels are arranged adjacent to one another, disposed to extend in a direction of the flow. Fluid passageways of the flow channels are overlapped partially on one another in a flow cell longitudinal direction of the flow cell body.