  It is known that relative entropy of entanglement for entangled state $\rho$
is defined via its closest separable (or positive partial transpose) state
$\sigma$. Recently, it has been shown how to find $\rho$ provided that $\sigma$
is given in two-qubit system. In this paper we study on the inverse process,
i.e. how to find $\sigma$ provided that $\rho$ is given. It is shown that if
$\rho$ is one of Bell-diagonal, generalized Vedral-Plenio and generalized
Horodecki states, one can always find $\sigma$ from a geometrical point of
view. This is possible due to the following two facts: (i) The Bloch vectors of
$\rho$ and $\sigma$ are identical with each other (ii) The qubit-interaction
vector of $\sigma$ can be computed from a crossing point between minimal
geometrical object, in which all separable states reside in the presence of
Bloch vectors, and a straight line, which connects the point corresponding to
the qubit-interaction vector of $\rho$ and the nearest vertex of the maximal
tetrahedron, where all two-qubit states reside. It is shown, however, that
these nice properties are not maintained for the arbitrary two-qubit states.
