  In order to resolve the long-standing issue of how the itinerant
ferromagnetism is affected by the lattice structure and Hund's coupling, we
have compared various three-dimensional lattice structures in the single- and
multiorbital Hubbard models with the dynamical mean-field theory with an
improved quantum Monte Carlo algorithm that preserves the spin-SU(2) symmetry.
The result indicates that {\it both} the lattice structure and the d-orbital
degeneracy are essential for the ferromagnetism in the parameter region
representing a transition metal. Specifically, (a) Hund's coupling, despite the
common belief, is important, which is here identified to come from
particle-hole scatterings, and (b) the ferromagnetism is a correlation effect
(outside the Stoner picture) as indicated from the band-filling dependence.
