  Overionized recombining plasmas (RPs) have been discovered from a dozen of
mixed- morphology (MM) supernova remnants (SNRs). However their formation
process is still under debate. As pointed out by many previous studies, spatial
variations of plasma temperature and ionization state provide clues to
understand the physical origin of RPs. We report on a spatially resolved X-ray
spectroscopy of W28, which is one of the largest MM SNRs found in our Galaxy.
Two observations with Suzaku XIS cover the center of W28 to the northeastern
rim where the shock is interacting with molecular clouds. The X-ray spectra in
the inner regions are well reproduced by a combination of two-RP model with
different temperatures and ionization states, whereas that in northeastern rim
is explained with a single-RP model. Our discovery of the RP in the
northeastern rim suggests an effect of thermal conduction between the cloud and
hot plasma, which may be the production process of the RP. The X-ray spectrum
of the north- eastern rim also shows an excess emission of the Fe I K{\alpha}
line. The most probable process to explain the line would be inner shell
ionization of Fe in the molecular cloud by cosmic-ray particles accelerated in
W28.
