  We study the production of gravitational waves from cosmic domain walls
created during phase transition in the early universe. We investigate the
process of formation and evolution of domain walls by running three dimensional
lattice simulations. If we introduce an approximate discrete symmetry, walls
become metastable and finally disappear. We calculate the spectrum of
gravitational waves produced by collapsing metastable domain walls.
Extrapolating the numerical results, we find the signal of gravitational waves
produced by domain walls whose energy scale is around 10^10-10^12GeV will be
observable in the next generation gravitational wave interferometers.
