  The field-induced Berezinskii-Kosterlitz-Thouless (BKT) transition in the
ground state of the triangular antiferromagnetic Ising model is studied by the
level-spectroscopy method. We analyze dimensions of operators around the BKT
line, and estimate the BKT point $H_{\rm c}\simeq0.5229\pm0.001$, which is
followed by a level-consistency check to demonstrate the accuracy of our
estimate. Further we investigate the anisotropic case to clarify the stability
of the field-induced string-density plateau against an incommensurate liquid
state by the density-matrix renormalization-group method.
