  The flavor-singlet H-dibaryon, which has strangeness -2 and baryon number 2,
is studied by the approach recently developed for the baryon-baryon
interactions in lattice QCD. The flavor-singlet central potential is derived
from the spatial and imaginary-time dependence of the Nambu-Bethe-Salpeter wave
function measured in N_f=3 full QCD simulations with the lattice size of L = 2,
3, 4 fm. The potential is found to be insensitive to the volume, and it leads
to a bound H-dibaryon with the binding energy of 30 - 40 MeV for the
pseudo-scalar meson mass of 673 - 1015 MeV.
