  The supersymmetric model of hybrid inflation is interesting not only because
of its naturalness but also because of another reason. Its energy scale
determined by the COBE normalization is 10^15 - 10^16 GeV. It happens to be the
energy scale of interest in particle physics, namely, the mass scale of
right-handed neutrinos or the energy scale of the gauge-coupling unification.
It is true that topological defects are produced after the hybrid inflation if
it is related to a U(1)_B-L or a GUT-symmetry breaking, and hence one cannot
naively identify models of particle physics with that of inflation. But those
defects are not necessarily found in modified models. We show in this article
that quite a simple extension of the minimal supersymmetric hybrid inflation
model is free from monopoles or cosmic strings. Moreover, it happens to be
exactly the same as a well-motivated extension of SU(5)-unified theories. The
vacuum energy is dominated by F-term. The \eta-problem is not necessarily
serious when the model is realized by D-branes. Although it has been considered
that a coupling constant has to be very small when the vacuum energy is
dominated by F-term, this constraint is not applied either to the D-brane
model. They are due to a particular form of the Kahler potential and
interaction of the model. Reheating process is also discussed.
