Before a fluorescent labeling agent is administered on an observation area, the observation area is irradiated with an excitation light for fluorescence quenching. An image taking unit (3) captures two fluorescence images at a predetermined time interval during the irradiation with the excitation light. The first fluorescence image is written to a first frame memory (17a), and a second fluorescence image is written to a second frame memory (17b). An image processing circuit (18) reads out the two fluorescence images, and calculates a difference in light intensity between the two fluorescence images. A judgment circuit (26) judges whether or not fluorescence from a fluorescent dye that has already existed in the observation area is quenched based on the difference in light intensity.
