  We analyse the classical decay process of unstable D-branes in superstring
theory using the boundary string field theory (BSFT) action. We show that the
solutions of the equations of motion for the tachyon field asymptotically
approach to T=x^0 and the pressure rapidly falls off at late time producing the
tachyon matter irrespective of the initial condition. We also consider the
cosmological evolution driven by the rolling tachyon using the BSFT action as
an effective action.
