The production of RANKL may be inhibited by orally ingesting a milk-derived basic protein. Thus, a RANKL production inhibitor containing 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 containing the RANKL production inhibitor; and a food/beverage and a feed each containing a specified amount of the RANKL production inhibitor and being characterized by inhibiting the production of RANKL have a remarkable effect of inhibiting the production of RANKL, can be ingested on a daily basis, and exhibit high safety even when being ingested over a long period.