A sensor-chip includes a sensor-portion having at least a metal-layer deposited on a surface of a dielectric-plate. A fluorescent-label binding-substance in an amount corresponding to the amount of a detection-target-substance in a liquid-sample binds onto the sensor-portion. The amount of the detection-target-substance is detected based on the amount of light generated by excitation of a fluorescent-label in the fluorescent-label binding-substance. A fluorescent-substance the charge state of which changes in the liquid-sample according to the pH of the liquid-sample, and which contains a plurality of fluorescent-dye-molecules enclosed by a material that transmits fluorescence output from the fluorescent-dye-molecules, is used as the fluorescent-label. With the fluorescent-label binding-substance bound to the sensor-portion, the fluorescent-label is attracted to the sensor-portion by adjusting the pH of the sample-liquid to neutralize the charge state of the fluorescent-substance, thereby attracting the fluorescent-label onto the sensor-portion. In this state, the amount of the detection-target-substance is detected.