To provide a method whereby an interaction interface of a biomolecular complex, e.g. a protein-protein complex can be strictly and simply determined as compared with the conventional methods.

In mobile hydrogens of one biomolecule among a plurality of biomolecules composing the composite are substituted with heavy hydrogens and at least 70% of mobile hydrogens are substituted with heavy hydrogens. Thereafter, a position of a mobile hydrogen which is contained in one biomolecule present within 10 from hydrogens contained in the other adjacent biomolecule and is subjected to cross saturation is specified by the cross saturation phenomenon, whereby the interfacial residue of the biomolecular composite is identified. Accordingly, the interaction interface of the composite can be more strictly determined as compared with the conventional methods.