  Prominent K-shell emission lines of neutral iron (hereafter, FeI-K) and
hard-continuum X-rays were found from molecular clouds (MCs) in the Sagittarius
B (Sgr B) region with the two separate Suzaku observations in 2005 and 2009.
The X-ray flux of FeI-K decreased in correlation to the hard-continuum flux by
factor of 0.4-0.5 in 4 years, which is nearly equal to the light-travelling
across the MCs. The rapid and correlated time-variability, the equivalent width
of FeI-K, and the K-edge absorption depth of FeI are consistently explained by
"X-ray echoes" due to the fluorescent and Thomson-scattering of an X-ray flare
from an external source. The required flux of the X-ray flare depends on the
distance to the MCs and the duration time. The flux, even in the minimum case,
is larger than those of the brightest Galactic X-ray sources. Based on these
facts, we conclude that the super-massive black hole, Sgr A*, exhibited a
big-flare about a few hundred years ago and the luminosity of higher than
4x10^39 erg s^{-1}. The "X-ray echo" from Sgr B, located at a few hundred
light-years from Sgr A*, now arrived at the Earth.
