  We study theoretically the electronic structure of topological nodal-line
semimetals. We show that, in the presence of a gap-opening spatially dependent
mass term that forms a domain wall, an in-gap charged localized mode emerges at
the domain wall. It turns out that such a domain wall is realized by
head-to-head (or tail-to-tail) bulk electric polarizations. The localized mode
has a topological origin, i.e., a topological confinement is realized, which is
understood by a semiclassical topological number defined in the semiclassical
momentum-real space. In contrast to previous studies, our study demonstrates a
topological confinement at the interface between two insulators without bulk
topological numbers. Moreover, the dispersion of the localized mode evolves
from gapless to gapped as the bulk bandgap increases, which means that its
conductivity is externally tunable. We discuss a possible experimental
realization of the stable, electrically-tunable charged domain wall.
