  The Casimir energy for massless scalar field which satisfies priodic boundary
conditions in two-dimensional domain wall background is calculated by making
use of general properties of renormalized stress-tensor. The line element of
domain wall is time dependent, the trace anomaly which is the nonvanishing
$T^{\mu}_{\mu}$ for a conformally invariant field after renormalization,
represent the back reaction of the dynamical Casimir effect.
