ABSTRACT: We present the results of the ASCA 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, large absorption at low energy below 4 keV and pronounced edge-structure at 7.1 keV. The X-ray image is found to be extended with the peak position offset to the Galactic center direction by about 1-2 arcminute from the core of the molecular cloud. This morphology, as well as the X-ray spectrum, is well reproduced by a model that a strong X-ray source located at the Galactic center side is irradiating the molecular cloud Sgr B2, and only the reflected and the fluorescent X-rays are in our line of sight. Thus Sgr B2 is a new type of X-ray object, which may be called as an ``X-ray reflection nebula''. Possible implications of the Galactic center activity related to this unique source are presented.