An aperture stop 10A includes a filter area 1 formed on a flat-plate substrate 23, and an aperture area 2 formed inside the filter area 1. The filter area 1 transmits infrared light and reduces or blocks the transmission of visible light. The aperture area 2 transmits light in the wavelength range corresponding to fluorescent light from an observed area of a subject and light in the wavelength range corresponding to illumination light to the subject. The aperture stop 10A does not reduce the light in the wavelength range corresponding to fluorescent light from the observed area of the subject, but variably reduces visible light since the area of the aperture area 2 is variable. Therefore, it becomes possible to simultaneously and clearly observe, with a simple method, a subject image formed by illumination light in the visible light band and an observed image formed by weak fluorescent light from the observed area of the subject in the infrared light band.
