  Two-dimensional (2D) massless Dirac electrons appear on a surface of
three-dimensional topological insulators. The conductivity of such a 2D Dirac
electron system is studied for strong topological insulators in the case of the
Fermi level being located at the Dirac point. The average conductivity
$\langle\sigma\rangle$ is numerically calculated for a system of length $L$ and
width $W$ under the periodic or antiperiodic boundary condition in the
transverse direction, and its behavior is analyzed by applying a finite-size
scaling approach. It is shown that $\langle\sigma\rangle$ is minimized at the
clean limit, where it becomes scale-invariant and depends only on $L/W$ and the
boundary condition. It is also shown that once disorder is introduced,
$\langle\sigma\rangle$ monotonically increases with increasing $L$. Hence, the
system becomes a perfect metal in the limit of $L \to \infty$ except at the
clean limit, which should be identified as an unstable fixed point. Although
the scaling curve of $\langle\sigma\rangle$ strongly depends on $L/W$ and the
boundary condition near the unstable fixed point, it becomes almost independent
of them with increasing $\langle\sigma\rangle$, implying that it asymptotically
obeys a universal law.
