  We investigate the thermodynamic properties of D-brane-anti-D-brane pairs and
non-BPS D-branes on the basis of boundary string field theory. We calculate the
finite temperature effective potential of N D-brane-anti-D-brane pairs in a
non-compact background and in a toroidal background. In the non-compact
background case, a phase transition occurs slightly below the Hagedorn
temperature, and the D9-anti-D9 pairs become stable. Moreover, the total energy
at the critical temperature is a decreasing function of N as long as the 't
Hooft coupling is very small. This leads to the conclusion that a large number
N of D9-anti-D9 pairs are created simultaneously near the Hagedorn temperature.
In the toroidal background case (M_{1,9-D} * T_{D}), a phase transition occurs
only if the Dp-anti-Dp pair is extended in all the non-compact directions, as
long as the 't Hooft coupling is very small. The total energy at the critical
temperature also decreases as N increases. We also calculate the finite
temperature effective potential of non-BPS D-branes, and we obtain similar
results. Then, we consider the thermodynamic balance between open strings on
these branes and closed strings in the bulk in the ideal gas approximation, and
conclude that the total energy is dominated by the open strings.
