A technique for imaging the distribution of molecular species in a living body is an important tool used in medical and biological research. Imaging of molecular species at the cellular level has been widely performed using a microscope and a molecular probe such as a molecular probe labeled with a fluorescence pigment or a chemiluminescence molecular probe. However, recently, there is a growing demand for devices for observing in vivo the distribution of molecular species of interest at the organ or whole-body level rather than the cellular level. For example, such an observation device allows the imaging of the distribution of target cancer cells labeled with a fluorescence probe in the body of a small living animal, such as a mouse, to monitor the growth of the target cancer cells over a fixed period of time, such as every day or every week. In a case where the growth of cancer cells in the body of an animal is monitored using a conventional device for cellular-level imaging, the animal is killed to stain or fluorescently-label cancer cells in a predetermined part of the body of the animal. In this case, the growth of cancer cells in the same individual cannot be monitored over a long period of time. For this reason, there is a demand for the development of a device capable of observing the distribution of molecular species in the body of a small living animal to obtain internal information about the body of the small animal.
As an exciting light-irradiating device for exciting fluorescence, one shown in FIG. 9 is known. As shown in FIG. 9, the exciting light-irradiating device has a filter wheel 8 and a multi-branched optical fiber bundle 16 to irradiate an object with light having a wavelength selected by the filter wheel 8 and the multi-branched optical fiber bundle 16. More specifically, light is emitted from a light source 2 such as a tungsten halogen lamp, collected by a lens 4 so as to enter an optical guide 6, and guided to a filter 10 mounted on the filter wheel 8 by the optical guide 6 so that only light passed through the filter 10 is guided to an entrance portion 16A of the multi-branched optical fiber bundle 16. Optical fibers constituting the multi-branched optical fiber bundle 16 are tied in a bundle at the entrance portion 16A, and are separated into four bundles at the position of a ring 16B provided on the way to a dark measurement chamber (not shown). The distal ends of these four optical fiber bundles are placed in the dark measurement chamber. The filter wheel 8 has a plurality of filters, and a desired excitation wavelength is selected by switching among these filters. Exciting light is guided by the multi-branched optical fiber bundle 16 to predetermined positions in the dark measurement chamber for measuring fluorescence. A device similar to the exciting light-irradiating device shown in FIG. 9 is also disclosed in U.S. Pat. No. 6,894,289.