  We investigate the ground state phase diagram of the S=1/2 two-leg $XXZ$ spin
ladder system with an isotropic interchain coupling. In this model, there is
the Berezinskii-Kosterlitz-Thouless transition which occurs at the XY-Haldane
and the XY-rung singlet phase boundaries. It was difficult to determine the
transition line using traditional methods. We overcome this difficulty using
the level spectroscopy method combined with the twisted boundary condition
method, and we check the consistency. We find out that the phase boundary
between XY phase and Haldane phase lies on the $\Delta=0$ line. And we show
that there exist two different XY phases, which we can distinguish
investigating a $XX$ correlation function.
