An assay apparatus is loaded with a sensor unit as surface plasmon resonance (SPR) biosensor, which includes a transparent dielectric medium and thin film. A first surface of the thin film is a thin film/dielectric interface. Its sensing surface causes reaction of sample fluid. An optical assay unit assays reaction of the sample fluid by detecting illuminating light reflected by the interface. Measured data from the optical assay unit is compared with a reference parameter, to evaluate suitability of the measured data for analysis. If lack of the suitability is estimated in the evaluation, a succeeding density of the sample fluid to be used in a succeeding evaluating assay is determined according to the measured data with the estimated lack of the suitability. The sample fluid is prepared at the succeeding density by mixing with diluent fluid, and then is used for the succeeding evaluating assay.