  The radiatively induced neutrino mass matrix is investigated on the basis of
an SU(5) SUSY model. In order to evade the proton decay, an ansatz based on a
discrete symmetry $Z_2$ is assumed: although, at the unification scale, we have
two types of superfields $\Psi_{L(\pm)}=\bar{5}_{L(\pm)}+10_{L(\pm)}$, which
are transformed as $\Psi_{L(\pm)} \to \pm \Psi_{L(\pm)}$ under the discrete
symmetry Z$_2$, the particles $\Psi_{L(+)}$ are decoupled after the SU(5)
symmetry is broken, so that our quarks and leptons belong to \Psi_{L(-)}. The
R-parity-violating terms for our quarks and leptons \Psi_{L(-)} are basically
forbidden under the symmetry Z_2. However, we assume that mixings between
members of \Psi_{L(+)} and those of \Psi_{L(-)} are in part caused after SU(5)
is broken. As a result, the R-parity-violating interactions are in part
allowed, so that the neutrino masses are radiatively generated, while the
proton decay due to the R-parity violating terms is still forbidden because the
term d_R^c d_R^c u_R^c has z=-1.
