Superstring theory is a promising theory which can unify all the forces and the matters in particle physics. A new multi-dimensional object which is called "D-brane" was found recently, which drastically changed our perspective on unified world. We may live on membrane-like hypersurface in higher dimensions ("braneworld scenario"), or we can create blackholes at particle accelarators, or the dynamics of quarks is shown to be equivalent to higher dimensional gravity theory. All of these 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, and ends with future roads for unified ultimate theory of our world.