Structural isomers in sc(Fv)2 compositions of anti-human Mpl antibody and humanized anti-human Mpl antibody were separated, and the obtained structural isomers were cleaved at their linkers to confirm that the structural isomers are of single chain diabody type and bivalent scFv type. In addition, the agonistic activities of these structural isomers were revealed to be significantly different. Furthermore, the present inventors discovered that the content ratio of the structural isomers in sc(Fv)2 compositions could be regulated by altering temperature, modifying lengths of the linkers of sc(Fv)2, or amino acids in their variable regions.