  We develop a fast and accurate calculation method for ionization degrees in
protoplanetary and circumplanetary disks including dust grains. We apply our
method to calculate the ionization degree of circumplanetary disks. It is
important to understand the structure and evolution of
protoplanetary/circumplanetary disks since they are thought to be the sites of
planet/satellite formation. The turbulence that causes gas accretion is
supposed to be driven by magnetorotational instability (MRI) that occurs only
when the ionization degree is high enough for magnetic field to be coupled to
gas. We calculate the ionization degrees in circumplanetary disks to estimate
the sizes of MRI-inactive regions. We properly include the effect of dust
grains because they efficiently capture charged particles and make ionization
degree lower. Inclusion of dust grains complicates the reaction equations and
requires expensive computation. In order to accelerate the calculation of
ionization reactions, we develop a semianalytic method based on the charge
distribution model proposed previously. This method enables us to study the
ionization state of disks for a wide range of model parameters. For a previous
model of circum-Jovian disk, we find that an MRI-inactive region covers almost
all regions even without dust grains. This suggests that the gas accretion
rates in circumplanetary disks are much smaller than previously thought.
