Osteoporosis is a serious disease caused by decreased bone mass. There is constant matrix remodeling in bones, by which bone formation is performed by osteoblastic cells, whereas bone resorption is accomplished by osteoclast cells. We investigated the effect of a Japanese apricot (Prunus mume SIBE. et ZUCC.) extract on the proliferation and osteoblastic differentiation in pre-osteoblastic MC3T3-E1 cells. An alkaline phosphatase (ALP) activity assay, cell proliferation assay, alizarin red staining and expression analysis of osteoblastic genes were carried out to assess the proliferation and osteoblastic differentiation. The water-soluble fraction of Prunus mume (PWF) increased the ALP activity, cell proliferation and mineralization. The gene expression of osteopontin and bone morphogenetic protein-2, which are markers in the early period of osteoblastic differentiation, were significantly enhanced by the PWF treatment. PWF therefore stimulated the proliferation and osteoblastic differentiation of cells and may have potential to prevent osteoporosis.