The present invention: directs a light beam (a) onto a prism (21) having a metal thin film (25) formed on a specific surface (23), changing the angle of incidence (?) relative to the metal thin film (25) in a state of full reflection; measures light emitted on the surface (25a)-side of the metal thin film (25) as a result of the reflection of the light beam (a) by the metal thin film (25); determines the angle of incidence (? 5) at which to direct the light beam (a) onto the metal thin film (25) on the basis of the measured variation in light intensity; adjusts the direction in which said light beam is directed so that the light beam (a) is directed onto the metal thin film (25) at the determined angle of incidence (? 5); and measures fluorescent light emitted at the surface (25a)-side of the metal thin film (25) while the light beam (a) is being directed in the adjusted direction.
