  The classical first law of thermodynamics for a Kerr-Newman black hole (KNBH)
is generalized to a law in quantum form on the event horizon. Then four quantum
conservation laws on the KNBH equilibrium radiation process are derived. The
Bekenstein-Hawking relation ${\cal{S}}={\cal{A}}/4$ is exactly established. It
can be inferred that the classical entropy of black hole arises from the
quantum entropy of field quanta or quasi-particles inside the hole.
