The development of methane hydrate (MH), which exists under the ocean floor has recently been brought to public attention. However the production technology has not yet been established. It is important to understand the decomposition phenomenon of MH or an investigation of the safety and the profitability of production systems. In this research, the gas hydrate decomposition rate in flowing water was measured using HCFC141b hydrate as a substitute for MH. When the water temperature was higher than the boiling point of the decomposition gas, it was observed that the decomposition gas increased the decomposition rate. Moreover the decomposition phenomenon was simulated by the lattice gas automaton method in order to establish the technique which analytically estimates the decomposition rate. The validity of the simulation method was shown by comparing the experiments. Furthermore, the formula between Reynolds number and Nusselt number was obtained, which express the heat transfer around the gas hydrate lump.