  We investigate where in protoplanetary disks magnetorotational instability
operates, which can cause angular momentum transport in the disks. We
investigate the spatial distribution of various charged particles and the
unstable regions for a variety of models for protoplanetary disks taking into
account the recombination of ions and electrons at grain surfaces, which is an
important process in most parts of the disks. We find that for all the models
there is an inner region which is magnetorotationally stable due to ohmic
dissipation. This must make the accretion onto the central star non-steady. For
the model of the minimum-mass solar nebula, the critical radius, inside of
which the disk is stable, is about 20 AU, and the mass accretion rate just
outside the critical radius is 10^{-7} - 10^{-6} M_{\odot} yr^{-1}. The stable
region is smaller in a disk of lower column density. Dust grains in
protoplanetary disks may grow by mutual sticking and may sediment toward the
midplane of the disks. We find that the stable region shrinks as the grain size
increases or the sedimentation proceeds. Therefore in the late evolutionary
stages, protoplanetary disks can be magnetorotationally unstable even in the
inner regions.
