High pressure and high temperature in situ X-ray diffraction experiments on the stability of superhydrous phase B have been conducted to clarify the host of water in the slabs descending into the lower mantle. We observed the decomposition reaction of superhydrous phase B into phase G (= phase D and F), periclase, and MgSiO3 perovskite at around 700 ~1100¿C and above 30 GPa, and the boundary of the decomposition reaction can be expressed as P(GPa) = -0.0037(¿0.0017)T(¿C) + 32.2(¿0.5). This reaction can occur in slabs descending into the uppermost part of the lower mantle.