We propose an inelastic neutron scattering experiment on the chopper spectrometer MERLIN, which studies spin dynamics of Fe-pnictide LaFeAsO0.49D0.51 (TN ~ 90 K). The purpose of this proposal is to determine quantitatively spin excitations of the antiferromagnet, which was assigned recently by us as the second ¿parent phase¿ in LaFeAsO1-xHx system. By exploring the overall magnetic excitations with respect to the high-energy magnetic dispersion relation and the low-energy spin gap with magnetic anisotropy, we will gain a deep insight on the bipartite, symmetric phase diagram from the spin dynamics viewpoint.