  We examine a possibility that, when a black hole is formed, the information
on the collapsed star is stored as the entanglement entropy between the outside
and the thin region (of the order of the Planck length) of the inside the
horizon. For this reason, we call this as the entanglement entropy of the black
hole ``horizon''. We construct two models, one is in the Minkowski spacetime
and the other is in the Rindler wedge. To calculate the entropy explicitly, we
assume that the thin regions of the order of the Planck length of the outside
and inside the horizon are completely entangled by quantum effects. We also use
a property of the entanglement entropy that it is symmetric under an
interchange of the observed and unobserved subsystems. Our setting and this
symmetric property substantially reduce the needed numerical calculation. As a
result of our analysis, we can explain the Bekenstein-Hawking entropy itself
(rather than its correction by matter fields) in the context of the
entanglement entropy.
