  Gravitational waves from cosmological double neutron star binaries can be
significantly demagnified by strong gravitational lensing effect, and the
proposed future missions such as BBO or DECIGO might miss some of the
demagnified GW signals below a detection threshold. The undetectable binaries
would form a GW foreground which might hamper detection of a very weak
primordial GW signal. We discuss the outlook of this potential problem, using a
simple model based on the singular-isothermal-sphere lens profile. Fortunately,
it is expected that, for presumable merger rate of NS+NSs, the residual
foreground would be below the detection limit Omega_{GW} ~ 10^{-16} realized
with BBO/DECIGO by correlation analysis.
