An illumination aperture diaphragm 100A which is used in a light source device to simultaneously observe a subject image formed by reflected illumination light in the visible light region and a fluorescence image from a fluorescent substance in a subject has a filter region 1 formed in a flat plate-like base material 3 and an aperture region 2 formed inside the filter region 1. The filter region 1 transmits light in the wavelength region of excitation light for allowing the subject to emit fluorescent light and reduces or blocks the transmission of the light in the visible light region that forms the subject image. An illumination aperture diaphragm 200A may include a plurality of diaphragm blade members 20A which are each provided partially or entirely with a filter portion 21, so that a filter region 1 is formed of the plurality of diaphragm blade members 20A and an aperture region 2 is formed inside the filter region 1. This can provide an improved ratio in the quantity of light between the subject image provided by the reflected illumination light and the weak fluorescence image provided by the fluorescent substance in the subject.
