  By developing an appropriate path-integral formalism, we compute, in bosonic
string theory, the disk amplitude for the scattering of closed string states
from a D-particle, in which the collective coordinate of the D-particle is
fully quantized. As a consequence, the recoil of the D-particle is naturally
taken into account. Our result can be readily factorized in the closed string
channel to yield the boundary state describing the recoiling D-particle. This
turned out to agree with the BRST invariant vertex recently proposed by
Ishibashi to the leading order in the derivative expansion, but it will receive
corrections in subsequent orders. The advantage of our formalism is that it is
extendable to deal with more general processes involving multiple D-particles.
A viewpoint regarding our work as describing a dynamical transition of CFT's is
also discussed.
