  We show that the planetesimal formation due to the gravitational
fragmentation of a dust layer in a protoplanetary disk is possible. The dust
density distribution in the dust layer would approach the constant Richardson
number distribution due to the dust stirring by the shear instability and dust
settling. We perform the analysis of the shear instability of dust layer in a
protoplanetary disk with the constant Richardson number density distribution.
Our study revealed that this distribution is stable against the shear
instability even if the dust density at the midplane reaches the critical
density of the gravitational instability, and the planetesimal formation
through the gravitational fragmentation of the dust layer can occur even for
the dust to gas surface density ratio with the solar composition.
