  I discuss my recent attempt in search of a new framework for quantum field
theory of D branes. After explaining some motivations in the background of this
project, I present, as a first step towards our goal, a second-quantized
reformulation of the U(N) Yang-Mills quantum mechanics in which the D0-brane
creation-and-annihilation fields connecting theories with different N are
introduced. Physical observables are expressed in terms of bilinear forms of
the D0 fields. The large N limit is briefly treated using this new formalism.
