Composed of methane gas and cage-like water molecules, methane hydrate is expected to have innovative engineering applications, such as gas transportation. In this study, a methane hydrate decomposition interface is visualized qualitatively, and the decomposition rate is discussed. To clarify the decomposition mechanism, consideration is given to the dynamic decomposition interface and the variation of the decomposition rate. In the present experiment, methane hydrate is formed around a water droplet and decomposed by depressurization in the gas phase, the dynamic variation of the decomposition interface is observed precisely using a high-resolution camera, and three different depressurization conditions are used to confirm the interfacial change. It is found that the decomposition rate and dynamic shape change of the decomposition interface depend on the difference between the equilibrium pressure and that of the gas phase. In addition, the decomposition time is discussed using the decomposition model, and it is estimated that the present experimental decomposition rate corresponds to a lower activation energy for decomposition compared with that of the model by other authors. It is assumed that the decomposition proceeds under nearly reaction-limited conditions. Additionally, the interfacial deformation and collapse are observed in the case of pressure reduction near the equilibrium pressure, and validation is provided by a numerical simulation considering the heat and mass transfer near the interface. The numerical results suggest that the decomposition is affected by the interfacial configuration and the nearby temperature and concentration distributions.