Superstring theory is a promising theory which can potentially unify all
the forces and the matters in particle physics. A new multi-dimensional
object which is called "D-brane" was found. It drastically changed our
perspective of a unified world. We may live on membrane-like
hypersurfaces in higher dimensions ("braneworld scenario"), or we can
create blackholes at particle accelarators, or the dynamics of quarks is
shown to be equivalent to the higher dimensional gravity theory. All
these scenarios are explained in this book with plain words but with
little use of equations and with many figures. The book starts with a
summary of long-standing problems in elementary particle physics and
explains the D-branes and many applications of them. It ends with future
roads for a unified ultimate theory of our world.