  We investigate a (semi-)metal to insulator transition (MIT) realized in
geometrically frustrated electron systems on the basis of the Hubbard model on
a three-dimensional pyrochlore lattice and a two-dimensional checkerboard
lattice. Using the renormalization group method and mean field analysis, we
show that in the half-filling case, MIT occurs as a result of the interplay
between geometrical frustration and electron correlation. In the insulating
phase, which has a spin gap, the spin rotational symmetry is not broken, while
charge ordering exists. The charge ordered state is stabilized so as to relax
the geometrical frustration in the spin degrees of freedom. We also discuss the
distortion of the lattice structure caused by the charge ordering. The results
are successfully applied to the description of the MIT observed in the
pyrochlore system ${\rm Tl_2Ru_2O_7}$.
