Provided is an assay method utilizing surface plasmon-field enhanced fluorescence spectroscopy, which suppresses fluctuations in an absolute value of a background signal that is referred to as a resonance angle or an optical blank by improving the effect when a whole blood sample is used, specifically, the effect by the blood cell components remaining in a measurement liquid during a SPFS measurement, with regards to a photometric condition setting step, which is a characteristic step in assay methods utilizing SPFS. The above-mentioned problem is solved by an assay method utilizing surface plasmon-field enhanced fluorescence spectroscopy, including a primary reaction step of a sample, a first washing step, a labeling reaction step, and a measurement step, wherein the method includes a step for evaluating a blood cell component quantity between the primary reaction step and the labeling reaction step.
