  We demonstrate that domain-wall type excitations carrying the fractional
magnetization $S^z=\pm1/3$ and their bound state describe the low-energy
physics around the 1/3 plateau of the antiferromagnetic zigzag spin chain in
the strongly frustrated region. In particular we discuss the relevance of such
domain-wall excitations to the characteristic properties of the magnetization
curves: the 1/3 plateau accompanying the spontaneous symmetry breaking of the
translation, the cusp singularities above and/or below the plateau, and the
even-odd effect in the magnetization curve.
