  We investigate how much the ratio of cosmic microwave background (CMB)
polarization power spectra $C^{BB}_\ell/C^{EE}_\ell$ at low-$\ell$ ($\ell
\lesssim 10$) depends on the process of reionization. Both such low-$\ell$
B-mode and E-mode polarization powers are dominantly produced by Thomson
scattering of CMB photons off the free electrons which are produced in the
process of reionization. Since the reionization should be finished until at
least the redshift $z \simeq 6$ and the low-$\ell$ polarization powers are
produced at late time, the ratio is rather insensitive by the ionization
process at higher redshifts, but it is sensitive to the value of optical
depth.The value of the ratio at $\ell=2$, however, is almost insensitive to the
reionization process including the value of optical depth, and the value is
approximately half of the value of tensor-to-scalar ratio. This fact can be
utilized for future determination of tensor-to-scalar ratio in spite of the
ambiguity due to cosmic variance.
