  We investigate entanglement of two electron spins forming Cooper pairs in an
s-wave superconductor. The two-electron space-spin density matrix is obtained
from the BCS ground state using a two-particle Green's function. It is
demonstrated that a two spin state is not given by a spin singlet state but by
a Werner state. It is found that the entanglement length, within which two
spins are entangled, is not the order of the coherence length but the order of
the Fermi wave length.
