  We investigate the stochastic gravitational wave background (SGWB) from
cosmic domain walls (DWs) caused by quantum fluctuations of a light scalar
field $\phi$ during inflation. Perturbations of $\phi$ remain almost constant
after leaving the Hubble horizon. The probabilities of the two domains depend
on the averaged value of $\phi$ at each large scale region, leading to large
scale perturbations of DW energy density and large anisotropies in the SGWB. We
find that the angular power spectrum is scale-invariant and at least of the
order of $10^{-2}$, which is expected to be detected by future
gravitational-wave (GW) detectors. Since we have not detected primordial GWs
yet, anisotropies of the SGWB could help us to verify the rationality of
inflation and determine the energy scale of inflation.
