A fluorescence viewing device 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 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, an exposure time of a fluorescence image acquisition period and an exposure time of a background image acquisition period are different from each other. Since ON/OFF durations of excitation light also become asymmetric according to the asymmetry of the exposure times, the device can reduce user's feeling of strangeness due to blinking of the excitation light.