In-situ observation of methane-hydrate (MH) formation and dissociation in sandy sediments, and water-permeability measurements for the sediments have been performed by the apparatus combined proton nuclear magnetic resonance (1H NMR) and permeability measurement system, which we had originally designed. For three kinds of sand, Toyoura, No. 7 and No. 8 silica sand, the changes of pore-size distributions under MH formation and dissociation processes were discussed from the changes of transverse relaxation time, T2, of liquid water in pore space. With forming MH, it was found that intensity and peak-top T2 of T2 distribution decreased. The intensity of T2 distribution is proportional to the number of protons of water and methane in pore space. Decreasing intensity means the consumption of water and methane to form MH, and the shorter peak-top T2 indicates smaller apparent pore size, in which MH was assumed to be solid material as well as sand grain. These changes are thought to indicate that MH growth make apparent pore size gradually smaller. On the other hand, MH dissociation is supposed to advance from water-hydrate interfaces, where apparent pore size becomes larger gradually. All the sediments used here have the same tendency of decreasing pore-size with changing water saturation due to MH formation and growth. Measured water permeability ratio, which is defined as the ratio of effective water permeability to absolute one, during MH dissociation increased rapidly in the case of higher MH saturation, Sh, condition and went up slowly at lower Sh condition, while that estimated from SDR model increased gradually for all Sh condition. But it was found that the expanded SDR model which was introduced an exponent factor to the logarithmic mean of T2 (T2LM) of original SDR model could express the change of measured permeability. Here, the expansion of T2LMmeans extremely larger specific surface area (SSA) of pore space at higher S SUB>h. We interpret that higher SSA bring stronger resistance to water mobility, which leads extremely smaller water permeability at higher S SUB>h.