A new technique fabricating single crystals and polycrystalline aggregates of methane hydrate at room temperature under high pressure was established using a diamond anvil cell. In-situ observations by optical microscopy and X-ray diffractometry revealed high-pressure behavior up to 5.5 GPa, including growth, a compression process associated with changes in cage occupancy, and decomposition into high-pressure ice and solid methane. Interesting features such as an amoeba-like motion at crystallization and sponge-like behavior with pressure changes were observed. Cage occupancy, the so-called hydration number, was estimated from the relative intensity of the X-ray diffraction pattern, and changes in cage occupancies dependent on pressure were clearly observed.