  The reducible K\"ahler coset space G/S\otimesU(1)^k is discussed in a
geometrical approach. We derive the formula which expresses the Riemann
curvature of the reducible K\"ahler coset space in terms of its Killing
vectors. The formula manifests the group structure of G. On the basis of this
formula we establish an algebraic method to evaluate the four-fermi coupling of
the supersymmetric non-linear sigma-model on G/S\otimesU(1)^k at the low-energy
limit. As an application we consider the supersymmetric non-linear sigma-model
on E_7/SU(5)\otimesU(1)^3 which contains the three families of {10} + {5^*} +
{1} of SU(5) as the pseudo NG fermions. The four-fermi coupling constants among
diffferent families of the fermions are explicitly given at the low-energy
limit.
