  Casimir interaction between topological insulators with opposite topological
magnetoelectric polarizabilities and finite surface band gaps has been
investigated. For large surface band gap limit$(m\rightarrow\infty)$, we can
obtain results given in [Phys. Rev. Lett. \textbf{106}, 020403 (2011)]. For
small surface band gap limit$(m\rightarrow0)$, Casimir interaction between
topological insulators is attractive and analogy to ideal mental in short
separation limit. Generally, there is a critical value $m_c$ and when the
surface band gap is greater than the critical value, the Casimir force is
repulsive in an intermediate separation region. We estimate the critical
surface band gap $m_c \sim 1/(2a)$, where $a$ is a critical separation where
Casimir force vanishes.
