  We study the Page curve of an evaporating black hole using a toy model given
by Jackiw-Teitelboim gravity with Fateev-Zamolodchikov-Zamolodchikov-Teschner
(FZZT) antibranes. We treat the anti-FZZT branes as dynamical objects, taking
their back-reaction into account. We construct the entanglement entropy from
the dual matrix model and study its behavior as a function of the 't Hooft
coupling $t$ proportional to the number of branes, which plays the role of
time. By numerical computation we observe that the entropy first increases and
then decreases as $t$ grows, reproducing the well-known behavior of the Page
curve of an evaporating black hole. The system finally exhibits a phase
transition, which may be viewed as the end of the evaporation. We study the
critical behavior of the entropy near the phase transition. We also make a
conjecture about the late-time monotonically decreasing behavior of the
entropy. We prove it in a certain limit as well as give an intuitive
explanation by means of the dual matrix model.
