  We performed time lag analysis on the X-ray light curves of Centaurus A (Cen
A) obtained by the Gas Slit Camera (GSC) aboard the Monitor of All-sky X-ray
Image ({\it MAXI}) in three energy bands (2--4 keV, 4--10 keV, and 10--20 keV).
We discovered a soft X-ray lag relative to higher energies (soft lag) on a
timescale of days in a flaring episode by employing the discrete correlation
function (DCF) and the z-transformed discrete correlation function (ZDCF)
method. In the episode, a peak and a centroid in the DCF and the ZDCF was
observed at a soft lag of $\sim 5$ days in 2--4 keV versus 4--10 keV and in
4--10 keV versus 10--20 keV, and $\sim 10$ days in 2--4 keV versus 10--20 keV.
We found it difficult to explain the observed X-ray variation by a single
energy injection with the one-zone synchrotron self-Compton (SSC) model, in
which the soft lags in these three energy bands reflect the different cooling
times of the relativistic electrons, by assuming the magnetic field and minimum
Lorentz factor estimated from a broad-band SED. Alternatively, if the
phenomenon is interpreted as cooling of Comptonizing electrons in a corona
covering the accretion disk, the temperature of the corona producing the
variable X-rays should be $\sim$10 keV for reconciliation with the soft lag in
the energy range of 2--20 keV.
