Adamantanediols are produced by the hydroxylation of adamantane compounds in a water/organic solvent two-phase system in the presence of a ruthenium compound and a hypochlorite. Throughout the hydroxylation, the hypochlorite concentration in the water phase is regulated within a narrow limited range, for example, by monitoring the pH of the reaction system. With such a control of the hypochlorite concentration, the adamantanediols are produced in a high selectivity and a high yield.