  We present the results of X-ray observations of the well-studied TeV blazar
Mrk 421 with the Suzaku satellite in 2006 April 28. During the observation, Mrk
421 was undergoing a large flare and the X-ray flux was variable, decreasing by
~ 50 %, from 7.8x10^{-10} to 3.7x10^{-10} erg/s/cm^2 in about 6 hours, followed
by an increase by ~ 35 %. Thanks to the broad bandpass coupled with
high-sensitivity of Suzaku, we measured the evolution of the spectrum over the
0.4--60 keV band in data segments as short as ~1 ksec. The data show deviations
from a simple power law model, but also a clear spectral variability. The
time-resolved spectra are fitted by a synchrotron model, where the observed
spectrum is due to a exponentially cutoff power law distribution of electrons
radiating in uniform magnetic field; this model is preferred over a broken
power law. As another scenario, we separate the spectrum into "steady" and
"variable" components by subtracting the spectrum in the lowest-flux period
from those of other data segments. In this context, the difference ("variable")
spectra are all well described by a broken power law model with photon index
Gamma ~ 1.6, breaking at energy epsilon_{brk} ~ 3 keV to another photon index
Gamma ~ 2.1 above the break energy, differing from each other only by
normalization, while the spectrum of the "steady" component is best described
by the synchrotron model. We suggest the rapidly variable component is due to
relatively localized shock (Fermi I) acceleration, while the slowly variable
("steady") component is due to the superposition of shocks located at larger
distance along the jet, or due to other acceleration process, such as the
stochastic acceleration on magnetic turbulence (Fermi II) in the more extended
region.
