  The method of probe brane is the powerful one to obtain the effective action
living on the probe brane from supergravity. We apply this method to the
unstable brane systems, and understand the tachyon condensation in the context
of the open/closed duality. First, we probe the parallel coincident branes by
the anti-brane. In this case, the mass squared of the stretched string becomes
negative infinite in the decoupling limit. So that the dual open string field
theory is difficult to understand. Next, we probe parallel coincident branes by
a brane intersecting with an angle. In this case, the stretched strings have
the tachyonic modes localized near the intersecting point, and by taking the
appropriate limit for the intersection angle, we can leave mass squared of this
modes negative finite in the decoupling limit. Then we can obtain the
information about the localized tachyon condensation from the probe brane
action obtained using supergravity.
