A surface plasmon resonance (SPR) assay apparatus has an assay stage, which is loadable with a sensor unit. The sensor unit has a prism, thin film, flow cell and flow channel for flow of sample. A multiple pipette assembly accesses the flow channel in the assay stage, and introduces the sample into the flow channel. A light source applies illuminating light to the thin film to satisfy a total reflection condition. A photo detector receives the illuminating light reflected by the thin film to output a signal. A pressing mechanism presses the sensor unit on the assay stage for holding in a direction of the access of the pipette assembly. The pressing mechanism includes a movable pad for contacting the sensor unit. A slider moves the pad to press the sensor unit. A spring plunger adjusts pressure of the pad to the sensor unit.