  We study the roughening transition of an interface in an Ising system on a 3D
simple cubic lattice using a finite size scaling method. The particular method
has recently been proposed and successfully tested for various solid on solid
models. The basic idea is the matching of the renormalization-group-flow of the
interface with that of the exactly solvable body centered cubic solid on solid
model. We unambiguously confirm the Kosterlitz-Thouless nature of the
roughening transition of the Ising interface. Our result for the inverse
transition temperature $K_R=0.40754(5)$ is almost by two orders of magnitude
more accurate than the estimate of Mon, Landau and Stauffer.
