The effectiveness of Ca supplements was evaluated to compensate for the lack of Ca intake in the Japanese people. To determine the relationship between dietary calcium intake and the rate of calcium accumulation in postmenopausal women with significant calcium loss, a crossover study was conducted using milk, calcium carbonate powder, and calcium citrate powder (UNICAL^TM) made from scallop shells. Calcium contents in urine, feces, and meals were measured to compare calcium accumulation rates ((intake−excretion)/intake). Results showed that UNICAL^TM, which is more soluble than calcium carbonate, was as effective as milk in reducing postmenopausal calcium loss. However, postmenopausal Japanese women were found to lack calcium even at 700 mg/day, above the standard recommended intake of 650 mg/day. These results suggest that further increasing the solubility of UNICAL^TM is very effective as a Ca supplement for Japanese people who are deficient in milk intake.