  Berezinskii-Kosterlitz-Thouless (BKT) transition is one of the instability
mechanisms for the Tomonaga-Luttinger liquid.
  But in the BKT transition, there are logarithmic-correction problems, which
make it very difficult to treat BKT transitions numerically.
  We propose a method, ``level spectroscopy'', to overcome such difficulties,
based on the renormalization group analysis and the symmetry consideration.
