Estimation of surface area of methane hydrate in sub-seabed sand sediment is important to predict the production rate of methane gas. The surface area depends on the morphology of hydrate in the sediment: i.e., how the methane hydrate exists in the sediment. First, we conducted heat-transfer experiment and numerical simulations with the same conditions of pressure and temperature, and confirmed that the thermal conductivity for the mixture of sand grains, water, gas, and hydrate follows a random model. Then, we conducted hydrate-dissociation experiments by depressurisation in a high-pressure cell packed with glass beads with changing porosity, hydrate saturation, and size of sediment particles. We proposed a model equation for specific surface area of hydrate in the sediment as a function of porosity, hydrate saturation, and the size of sediment particles. We also conducted numerical simulations of the heat transfer, using numerical models both for the dissociation of hydrate and the thermal conductivity of gas–water–sand–hydrate mixture. As a result, the proposed model equation for the specific surface area of hydrate was performed well in a validation by comparing the temporal changes of measured total gas production and its rate. We believe that this hydrate surface-area model can be applied to the numerical prediction of gas production from hydrate-bearing sediments.