This proposal describes an in situ muSR experiment on a hydrogen storage material, MgH2. This is because MgH2 attracts a special attention due to the fact that the hydrogen desorption temperature (Td) is drastically decreased by milling and additives. In order to clarify a key factor to determine Td, we have measured muSR spectra for "as-prepared" MgH2 and "milled with/without additives". We have found that rapid diffusion of liberated hydrogen in MgH2 is essential to accelerate the desorption reaction from the systematic change in the spin-lattice relaxation rate with milling and additives. In order to confirm our finding, however, we need to demonstrate that the muSR parameters changes simultaneously with the hydrogen desorption reaction. For such purpose, we want to measure pressure in the sample cell during the muSR experiment.