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 100 if the dust grains can grow up to 10m .