  The archival XMM-Newton data of the central region of M31 were analyzed for
diffuse X-ray emission. Point sources with the 0.5--10 keV luminosity exceeding
$\sim 4 \times 10^{35}$ erg s$^{-1}$ were detected. Their summed spectra are
well reproduced by a combination of a disk black-body component and a
black-body component, implying that the emission mainly comes from an assembly
of luminous low-mass X-ray binaries. After excluding these point sources,
spectra were accumulated over a circular region of $6\arcmin$ (1.2 kpc)
centered on the nucleus. In the energy range above 2 keV, these residual
spectra are understood mainly as contributions of unresolved faint sources and
spill-over of photons from the excluded point sources. There is in addition a
hint of a $\sim 6.6$ keV line emission, which can be produced by a hot
(temperature several keV) thin-thermal plasma. Below 2 keV, the spectra involve
three additional softer components expressed by thin-thermal plasma emission
models, of which the temperatures are $\sim 0.6$, $\sim 0.3$, and $\sim 0.1$
keV. Their 0.5--10 keV luminosities within 6$\arcmin$ are measured to be $\sim
1.2 \times 10^{38}$ erg s$^{-1}$, $\sim 1.6 \times 10^{38}$ erg s$^{-1}$, and
$\sim 4 \times 10^{37}$ erg s$^{-1}$ in the order of decreasing temperature.
The archival Chandra data of the central region of M31 yielded consistent
results. By incorporating different annular regions, all the three softer
thermal components were confirmed to be significantly extended. These results
are compared with reports from previous studies. A discussion is presented on
the origin of each thermal emission component.
