The present inventor discovered stabilizing agents/stabilizing conditions for suppressing isomerization reactions of sc(Fv)2. It was also discovered that the above-mentioned isomerization reactions can be suppressed through use of freeze-dried formulations. As disclosed herein, by applying the discovered stabilizing agents/stabilizing conditions or the freeze-dried formulation, the isomerization reaction of an sc(Fv)2-type molecule from the bivalent scFv type to the single chain diabody type, and/or the isomerization reaction from a single chain diabody type to a bivalent scFv type can be suppressed in both directions or one direction.