A surface plasmon resonance (SPR) assay apparatus is loaded with a sensor unit. The sensor unit has a sensing surface and a flow channel, which has an entrance end opening and an exit end opening, and causes analyte fluid introduced through the entrance end opening to flow on the sensing surface. An optical assay unit assays reaction of the analyte fluid on the sensing surface by detecting attenuation of the illuminated light reflected by a thin film/dielectric interface. A fluid dispenser introduces the analyte fluid to the entrance end opening by pipetting. A fluid collecting vessel stores the analyte fluid used and exited from the exit end opening. A drain conduit extends from the exit end opening toward the fluid collecting vessel, for passage of the used analyte fluid. A suction pump drains the used analyte fluid into the fluid collecting vessel through the drain conduit.