  Numerical studies on Mott transitions caused by the control of the ratio
between bandwidth and electron-electron interaction ($U$) are reported. By
using the recently proposed path-integral renormalization group(PIRG)
algorithm, physical properties near the transitions in the ground state of
two-dimensional half-filled models with the nearest and the next-nearest
neighbor transfers ($-t$ and $t'$, respectively) are studied as a prototype of
geometrically frustrated system. The nature of the bandwidth-control
transitions shows sharp contrast with that of the filling-control transitions:
First, the metal-insulator and magnetic transitions are separated each other
and the metal-insulator (MI) transition occurs at smaller $U$, although the
both transition interactions $U$ increase with increasing $t'$. Both
transitions do not contradict the first-order transitions for smaller $t'/t$
while the MI transitions become continuous type accompanied by emergence of
{\it unusual metallic phase} near the transition for large $t'/t$. A
nonmagnetic insulator phase is stabilized between MI and AF transitions. The
region of the nonmagnetic insulator becomes wider with increasing $t'/t$. The
phase diagram naturally connects two qualitatively different limits, namely the
Hartree-Fock results at small $t'/t$ and speculations in the strong coupling
Heisenberg limit.
