  We compute the boundary state associated with a moving D$p$-brane in the
presence of the open string tachyon field as a background field. The effect of
the tachyon condensation on the boundary state will be discussed. It leads to a
boundary state associated with a lower dimensional moving D-brane or a
stationary instantonic D-brane. The former originates from condensation along
the spatial directions and the latter comes from the temporal direction of the
D-brane's worldvolume. Using the boundary state we also study the interaction
amplitude between two arbitrary D$p_1$ and D$p_2$-branes. The long range
behavior of the amplitude is investigated which shows an obvious deviation from
the conventional form, due to the presence of the background tachyon field.
