  We analyze the stability of dust layer in protoplanetary disk to understand
the effect of the relative motion between gas and dust. The previous analyses
not including the effect of relative motion between gas and dust show that the
shear-induced turbulence may prevent the dust grains from settling sufficiently
to be gravitationally unstable. We determine the growth rate of
Kelvin-Helmholtz instability in wide range of parameter space, and propose a
possible path toward the planetesimal formation through the gravitational
instability. We expect the density of dust layer becomes
$\rho_\mathrm{d}/\rho_\mathrm{g} \sim 200$ if the dust grains can grow up to
5m.
