  A \pi-electron network in nanographite systems with zigzag edges exhibits
strongly localized edge states, which are expected to have peculiar properties.
We study effects of electron-electron interactions on ground-state properties
of the systems by means of the weak-coupling renormalization group and the
density-matrix renormalization-group method. It is shown that the ground state
is a spin-singlet Mott insulator with finite charge and spin gaps. We also find
that the edge states are robust against the electronic correlations, resulting
in edge effective spins which can flip almost freely. The schematic picture for
the low-energy physics of the systems is discussed.
