  We discuss the open string one-loop partition function in tachyon
condensation background of a unstable D-brane system. We evaluate the partition
function by using the boundary state formulation and find that it is in
complete agreement with the result obtained in the boundary string field
theory. It suggests that the open string higher loop diagrams may be produced
consistently by a closed string field theory, where the D-brane plays a role of
source for the closed string field.
