  We estimate the number of old, radiative supernova remnants (SNRs) detectable
in HI 21-cm emission line in the Galaxy. According to our result, the
contamination due to the Galactic background HI emission limits the number of
visible SNRs to 270, or 9% of the total HI SNRs. The telescope sensitivity
further limits the number. We compare the result with observations to find that
the observed number (< 25) of HI SNRs is much less than the expected. A
plausible explanation is that previous observational studies, which were made
toward the SNRs identified mostly in radio continuum, missed most of the HI
SNRs because they are too faint to be visible in radio continuum. We propose
that the faint, extended HI 21-cm emission line wings protruding from the
Galactic background HI emission in large-scale (l, v) diagrams could be
possible candidates for HI SNRs, although our preliminary result shows that
their number is considerably less than the expected in the inner Galaxy. We
conclude that a possible explanation for the small number of \schi SNRs in the
inner Galaxy is that the interstellar space there is largely filled with a very
tenuous gas as in the three-phase ISM model.
