  We present the ASCA results of imaging spectroscopy of the giant molecular
cloud Sgr B2. The X-ray spectrum is found to be very peculiar; it exhibits a
strong emission line at 6.4 keV, a low energy cutoff below about 4 keV and a
pronounced edge-structure at 7.1 keV. The X-ray image is extended and its peak
position is shifted from the core of the molecular cloud toward the Galactic
center by about 1--2 arcminute. The X-ray spectrum and the morphology are well
reproduced by a scenario that X-rays from an external source located in the
Galactic center direction are scattered by the molecular cloud Sgr B2, and come
into our line of sight. Thus Sgr B2 may be called an X-ray reflection nebula.
Possible implications of the Galactic center activity related to this unique
source are presented.
