  We study dynamics of Type IIB bound-state of a Dirichlet string and n
fundamental strings in the background of N fundamental strings. Because of
supergravity potential, the bound-state string is pulled to the background
fundamental strings, whose motion is described by open string rolling radion
field. The string coupling can be made controllably weak and, in the limit $1
<< g^2_{\rm st} n << g^2_{\rm st} N$, the bound-state energy involved is small
compared to the string scale. We thus propose rolling dynamics of open string
radion in this system as an exactly solvable analog for rolling dynamics of
open string tachyon in decaying D-brane. The dynamics bears a novel feature
that the worldsheet electric field increases monotonically to the critical
value as the bound-state string falls into the background string. Close to the
background string, D string constituent inside the bound-state string decouples
from fundamental string constituents.
