: Osteoporosis is a common metabolic bone disease often affecting postmenopausal women, and various medicines are available that attempt to treat and/or prevent this disease. Hachimi-jio-gan is a traditional oriental (Kampo) medicine used clinically, but it's mode of action is still unknown. In this study, the effects of Hachimi-jio-gan extract on intestinal calcium absorption were evaluated in an osteoporosis model using ovariectomized (OVX) and sham-operated (SHAM) mice by investigating pharmacokinetic parameters in these animals. The ability of Hachimi-jio-gan extract to inhibit lipopolysaccharide-mediated stimulation of osteoclasts in bones using receptor activator of the NF-kB ligand (RANKL)-induced osteoclast differentiation of RAW264.7 cells was also investigated. The metabolic behavior of calcium did not differ between OVX and SHAM mice as the pharmacokinetics of calcium was equivalent after intravenous administration. The absolute bioavailability of calcium indicated that the extent of intestinal calcium absorption in the OVX (10.3%) and SHAM (10.7%) mice was at similar low levels. Hachimi-jio-gan extract potentially improved the intestinal calcium absorption by 1.96-and 1.86-fold, respectively. Hachimi-jio-gan extract further suppressed the potent stimulation of RANKL-induced osteoclast differentiation in RAW264.7 cells. These results suggest that Hachimi-jio-gan extract may be suitable for treatment and prevention of osteoporosis through its ability to increase intestinal calcium absorption and suppress osteoclast differentiation.