511 keV Gamma Ray from Moduli Decay in the Galactic Bulge

We show that the $e^{+}+e^{-}$ decay of a light scalar boson of mass 1-10 MeV may account for the fluxes of 511 keV gamma ray observed by SPI/INTEGRAL. We argue that candidates of such a light scalar boson is one of the string moduli or a scalar partner of the axion in a supersymmetric theory.