  In this paper, we studied the nonmagnetic disorder effects onto the quantum
critical point (QCP) which intervenes an ordinary insulator and the
3-dimensional $Z_2$ quantum spin Hall insulator. The minimal model describing
this QCP is the single-copy of the 3+1 Dirac fermion, whose topological mass
$m$ induces the quantum phase transition. We derived the phase diagram spanned
by this mass-term $m$, chemical potential $\mu$ and strength of the disorder
within the self-consistent Born approximation. To infer the structure of the
low-energy effective theory, we further calculated the weak localization (WL)
correction to the conductivity. By way of this, we have found that the diffuson
consists of the two quasi-degenerate contributions having the diffusion pole;
one always behaves as the diffusion mode. The other becomes the massless mode
only at $m=0$. Based on this "two-mode picture", we will discuss the possible
microscopic picture of the "levitation and pair annihilation" phenomena,
recently discovered by Onoda et al.
