To provide an osteoclast differentiation inhibitor that allows increase of bone mass.SOLUTION: An osteoclast differentiation inhibitor has a flavonoid derivative represented by the formula (1) or (2) or a salt thereof. The osteoclast differentiation inhibitor can be used as a bone mass increasing agent, and, for example, can increase bone mass or bone density in a subject of administration.SELECTED DRAWING: Figure 1【課題】骨量増強可能な破骨細胞の分化抑制剤の提供。【解決手段】破骨細胞の分化抑制剤は、下記式(1)または(2)で表されるフラボノイド誘導体またはその塩を含む。破骨細胞の分化抑制剤は、骨量増強剤として使用でき、例えば、投与対象における骨量または骨密度を増強できる。【選択図】図1