  We report on the variations of the physical parameters of the jet observed in
the blazar Mrk 421, and discuss the origin of X-ray flares in the jet, based on
the analysis of the several spectral energy distributions (SEDs). The SEDs are
modeled using the one-zone synchrotron self-Compton (SSC) model and its
parameters determined using a Markov chain Monte Carlo method. The lack of data
at TeV energies means many of the parameters cannot be uniquely determined and
are correlated. These are studied in detail. We found that the optimal solution
can be uniquely determined only when we apply a constraint to one of four
parameters: the magnetic field (B), Doppler factor, size of the emitting
region, and normalization factor of the electron energy distribution. We used
31 sets of SED from 2009 to 2014 with optical-UV data observed with UVOT/Swift
and the Kanata telescope, X-ray data with XRT/Swift, and gamma-ray data with
the Fermi Large Area Telescope (LAT). The result of our SED analysis suggests
that, in the X-ray faint state, the emission occurs in a relatively small area
(~ 10^16 cm) with relatively strong magnetic field (B~10^-1 G). The X-ray
bright state shows a tendency opposite to that of the faint state, that is, a
large emitting area (~10^18 cm), probably in the downstream of the jet and weak
magnetic field (B~10^-3 G). The high X-ray flux was due to an increase in the
maximum energy of electrons. On the other hand, the presence of two kinds of
emitting areas implies that the one-zone model is unsuitable to reproduce, at
least a part of the observed SEDs.
