sc (Fv) 2 site-variant

The present inventors have found that in order to solve the above problem, by introducing a site-specific mutation in the sc (Fv) 2, was examined stabilizing effect of sc (Fv) 2. As a result, it succeeded in greatly improving the Tm value of the sc (Fv) 2 by first amino acid substitutions according to the present invention. Further, by introducing a site-specific mutation in sc (Fv) 2, sc (Fv) 2 has been found to stabilize.