  In this paper, following the previous study, we evaluate the spectrum of
gravitational wave background generated by domain walls which are produced if
some discrete symmetry is spontaneously broken in the early universe. We apply
two different methods to calculate the gravitational wave spectrum: One is to
calculate the gravitational wave spectrum directly from numerical simulations,
and another is to calculate it indirectly by estimating the unequal time
anisotropic stress power spectrum of the scalar field. Both analysises indicate
that the slope of the spectrum changes at two characteristic frequencies
corresponding to the Hubble radius at the decay of domain walls and the width
of domain walls, and that the spectrum between these two characteristic
frequencies becomes flat or slightly red tilted. The second method enables us
to evaluate the GW spectrum semi-analytically for the frequencies which can not
be resolved in the finite box lattice simulations, but relies on the
assumptions for the unequal time correlations of the source.
