  In this paper, we construct center stable manifolds around unstable line
solitary waves of the Zakharov--Kuznetsov equation on two dimensional
cylindrical spaces with $2\pi L$ period. In the previous paper, center stable
manifolds around unstable line solitary waves have been constructed without
critical speed $c =4n^2/5L^2$ for positive integer $n>1$. Since the linearized
operator around line solitary waves with critical speed is degenerate, we prove
the stability condition of the center stable manifold for critical speed by
applying to the estimate of 4th order term of a Lyapunov function.
