  A model with a scalar type eight-fermion interaction is investigated in
curved spacetime. The ground state of the model can be obtained by observing
the effective potential. Applying the Riemann normal coordinate expansion, we
calculate an effective potential of the model in a weakly curved spacetime. The
result is extended to models with multi-fermion interactions. We numerically
show the behavior of the effective potential and find the phase structure of
the model.
