A configuration was adopted, in which the orientations of free molecules and binding molecules within a solution are switched by switching the vibration direction of orientation controlling light, thereby switching the amount of light emitted by each free molecule and each binding molecule. There is a difference in the amounts of time required for the orientations of the free molecules and the binding molecules to switch accompanying the switch in the emission direction of the orientation controlling light. Therefore, the timings at which the amounts of light emitted by each type of molecule increase differ. Accordingly, the fluorescence contributed by fluorescent molecules associated with free molecules and the fluorescent molecules associated with binding molecules can be respectively calculated, even if all of the fluorescent molecules within the solution emit fluorescence. Thereby, the concentration of a detection target substance can be accurately measured with a simple structure.