A fluorescence indicator is injected into a living body from a container via an injecting inlet to be chemically reacted with a substance in the living body. Then, an excitation light is irradiated to the living body from an excitation light source. Then, the thus obtained reflected light is reflected multiply at an reflective film formed on the inner wall of the container, and introduced into a detecting section, where a fluorescence response originated from the chemical bond between the substance and the fluorescence indicator and contained in the reflected light is detected.