  We found simultaneous decrease of Fe-K line and 4.2-6 keV continuum of
Cassiopeia A with the monitoring data taken by Chandra in 2000-2013. The flux
change rates in the whole remnant are $-$0.65$\pm$0.02 \% yr$^{-1}$ in the
4.2--6.0 keV continuum and $-$0.6$\pm$0.1 \% yr$^{-1}$ in the Fe-K. In the
eastern region where the thermal emission is considered to dominate, the
variations show the largest values: $-$1.03$\pm$0.05 \% yr$^{-1}$ (4.2-6 keV
band) and $-$0.6$\pm$0.1 \% yr$^{-1}$ (Fe-K line). In this region, the time
evolution of the emission measure and the temperature have a decreasing trend.
This could be interpreted as the adiabatic cooling with the expansion of $m =
0.66$. On the other hand, in the non-thermal emission dominated regions, the
variations of the 4.2--6 keV continuum show the smaller rates: $-$0.60$\pm$0.04
\% yr$^{-1}$ in the southwestern region, $-$0.46$\pm$0.05 \% yr$^{-1}$ in the
inner region and $+$0.00$\pm$0.07 \% yr$^{-1}$ in the forward shock region. In
particular, the flux does not show significant change in the forward shock
region. These results imply that a strong braking in the shock velocity has not
been occurring in Cassiopeia A ($<$ 5 km s$^{-1}$ yr$^{-1}$). All of our
results support that the X-ray flux decay in the remnant is mainly caused by
the thermal components.
