  It has been pointed out that the spurious Cosmic Microwave Background (CMB)
B-mode polarization signals caused by the absorption of the CMB monopole
component due to the Galactic interstellar matter, called the CMB shadow,
degrade the accuracy of detecting the CMB B-mode polarization signals imprinted
by primordial gravitational waves. We have made a realistic estimation using
simulated sky maps of how the CMB shadow affects forthcoming high-precision CMB
B-mode experiments for the first time. The Delta-map method, an internal
template method taking into account the first-order spatial variation of
foregrounds' spectral parameters, is applied as a foreground removal method. We
show that if the CMB shadow effects are not taken into account in the
foreground removal process, future observations would lead to the false
detection of the CMB B-mode polarization signals originating from primordial
gravitational waves. We also show that the effect of the CMB shadow can be
mitigated by our revised Delta-map method to target the CMB B-mode polarization
signals at the level of tensor-to-scalar ratio r=0.001.
