  The recently observed pentaquark baryons $\Theta^{+}$ and $\Xi^{--}$ are
studied in the dual gravity theory of QCD.
  By developing a general formulation useful for studying the branched string
web in a curved space, simple mass formulae of the pentaquark baryons are
derived in the Maldacena prototype model for supersymmetric QCD and a more
realistic model for ordinary QCD. Even though the approximations adopted here
in deriving these formulae are extremely naive, the predicted values of the
masses do not differ significantly from their experimental values. We thus
conclude that this approach is promising for the purpose of obtaining a
description of the observed masses of pentaquarks and of their extremely narrow
decay widths. With the aim of constructing more reliable mass formulae, a
preliminary sketch is given of how spin, the hyperfine interaction and parity
are considered in the string picture.
