A fluorescence viewing device 1A is configured to take a difference between one of an image of a first frame and an image of a second frame output from an imaging device 12 and the other of the first frame image and the second frame image stored in an image storage means, whereby clear observed images can be obtained without influence of background light. In the fluorescence viewing device 1A, an exposure time of a fluorescence image acquisition period and an exposure time of a background image acquisition period are different from each other. When the exposure time of the fluorescence image acquisition period and the exposure time of the background image acquisition period are made asymmetric in this manner, the observed images are acquired with good smoothness. Since ON/OFF durations of excitation light also become asymmetric according to the asymmetry of the exposure times, the device can reduce users feeling of strangeness due to blinking of the excitation light.
