A sensor unit of a surface plasmon resonance (SPR) assay system includes a transparent dielectric medium. A thin film has a first surface and a sensing surface. The first surface is connected with the dielectric medium to constitute an interface. The sensing surface is back to the first surface, for detecting (bio) chemical reaction. A flow cell block has a flow channel for flowing of the sample to the sensing surface. Attenuated total reflection of illuminating light is checked at the interface, to analyze interaction between ligand and analyte as samples. The flow channel includes a first inner surface, disposed opposite to the sensing surface to extend along, for passing the sample to flow between. The first inner surface has a height, defined with reference to the sensing surface, and in a range of 200-500 microns.