The production of RANKL can be inhibited by orally ingesting a milk-derived basic protein. Thus, disclosed are: a RANKL production inhibitor comprising a milk-derived basic protein fraction as an active ingredient； a therapeutic agent for a metabolic bone disease and a therapeutic agent for an immune disease each comprising the RANKL production inhibitor； and a food/beverage and a feed each of which contains a specified amount of the RANKL production inhibitor and can inhibit the production of RANKL. These products can significantly inhibit the production of RANKL, can be ingested on a day-to-day basis, and is highly safe even when ingested over a long period.
