The present inventor discovered stabilizing agents/stabilizing conditions forsuppressingisomerization reactions of sc(Fv)2. It was also discovered that the above-mentionedisomerization reactions can be suppressed through use of freeze-driedformulations. Asdisclosed herein, by applying the discovered stabilizing agents/stabilizingconditions or thefreeze-dried formulation, the isomerization reaction of an sc(Fv)2-typemolecule from thebivalent scFv type to the single chain diabody type, and/or the isomerizationreaction from asingle chain diabody type to a bivalent scFv type can be suppressed in bothdirections or onedirection.