Gas hydrate is one of the new energy resources because of vast amount of reservoirs in marine sediments. Recently, research and developments for the gas hydrate production have been conducted in some countries. To evaluate the production from the reservoirs, it is necessary to understand the behavior of dissociation in-situ condition. However, the rate of dissociation of gas hydrate has been unknown, especially in the situation of pore space of marine sediments. In this paper, the rate of dissociation of methane hydrates occupied in pore space is discussed. Based on the microscopic observation using Raman spectrometer, the mechanism of dissociation in pore space has been clarified. The rates of gas hydrate dissociation were measured under various unstable conditions, such as pressure decrease and heat stimulation. Dissociation model to simulate the interaction between hydrate crystals and pore space was considered on the basis of the experimental results. Through the above discussion, the estimation method for dissociation rate of methane hydrates was developed and validated. The simulation results using the developed model agree well with the data obtained from bulk-scale experiments.