  We discuss a rate asymmetry for the radiative $B$-meson decay $B \to X_s
\gamma$ within the framework of the supersymmetric standard model based on N=1
supergravity. This model contains new sources of CP violation. In spite of
severe experimental constraints on the electric dipole moment of the neutron, a
new CP-violating phase may not be suppressed, leading to a sizable enhancement
of the decay rate asymmetry. The magnitude of the asymmetry is predicted to be
larger than that by the standard model in wide parameter regions where the
branching ratio is consistent with its experimental bounds. A possible maximal
magnitude is about 0.1, which will be well accessible at B factories.
