  Whether spin-independent Coulomb interaction can be the origin of a realistic
ferromagnetism in an itinerant electron system has been an open problem for a
long time. Here we study a class of Hubbard models on decorated lattices, which
have a special property that the corresponding single-electron Schr\"{o}dinger
equation has $N_{\rm d}$-fold degenerate ground states. The degeneracy $N_{\rm
d}$ is proportional to the total number of sites $\abs{\Lambda}$. We prove that
the ground states of the models exhibit ferromagnetism when the electron
filling factor is not more than and sufficiently close to $\rho_0=N_{\rm
d}/(2\abs{\Lambda})$, and paramagnetism when the filling factor is sufficiently
small. An important feature of the present work is that it provides examples of
three dimensional itinerant electron systems which are proved to exhibit
ferromagnetism in a finite range of the electron filling factor.
