  We have estimated the critical exponent describing the divergence of the
localization length at the metal-quantum spin Hall insulator transition. The
critical exponent for the metal-ordinary insulator transition in quantum spin
Hall systems is known to be consistent with that of topologically trivial
symplectic systems. However, the precise estimation of the critical exponent
for the metal-quantum spin Hall insulator transition proved to be problematic
because of the existence, in this case, of edge states in the localized phase.
We have overcome this difficulty by analyzing the second smallest positive
Lyapunov exponent instead of the smallest positive Lyapunov exponent. We find a
value for the critical exponent $\nu=2.73 \pm 0.02$ that is consistent with
that for topologically trivial symplectic systems.
