Recently, various physiological effects of the tea polyphenol catechin for alleviating diseases such as cancer, arteriosclerosis, hyperlipidaemia and osteoporosis have been reported. However, the physiological effect of catechin on bone metabolism remains unclear. We examined the physiological effect of EGCG [(−)-epigallocatechin-3-gallate], which is the main component of green tea catechin, on osteoblast development using the precursor cell line of osteoblasts, MC3T3-E1, and co-culture of the osteoblasts from mouse newborn calvaria and mouse bone marrow cells. Although EGCG did not affect the viability and proliferation of MC3T3-E1 cells, EGCG inhibited the osteoblast differentiation. Furthermore, EGCG did not affect the mineralization of differentiated MC3T3-E1 cells, and reduced osteoclast formation in co-culture. These results suggest that EGCG can effectively suppress bone resorption, and can be used as an effective medicine in the treatment of the symptoms of osteoporosis.