Disclosed relates to organic germanium nanocolloids, to which technetium-99m (.sup.99mTc), a radionuclide, is labeled, method of the same, and a spleen-imaging agent including the same. According to the invention, .sup.99mTc-labeled organic germanium nanocolloids having high labeling efficiency and stability can be provided. Moreover, since the .sup.99mTc-labeled organic germanium nanocolloids of the invention are accumulated in the spleen considerably higher than the conventional spleen-imaging agent, it is possible to use the .sup.99mTc-labeled organic germanium nanocolloids of the present invention as a therapeutic radiopharmaceutical for the spleen imaging.