  In this thesis properties and the origin of black hole entropy are
investigated from various points of view. First, laws of black hole
thermodynamics are reviewed. In particular, the first and generalized second
laws are investigated in detail. It is in these laws that the black hole
entropy plays key roles. Next, three candidates for the origin of the black
hole entropy are analyzed: the D-brane statistical-mechanics, the brick wall
model, and the entanglement thermodynamics. Finally, discussions are given on
semiclassical consistencies of the brick wall model and the entanglement
thermodynamics and on the information loss problem.

General Relativity and Quantum Cosmology;High Energy Physics - Theory