The dissociation conditions of methane hydrates in confined small pores were measured by the gradual temperature increase method. Significant downward shifts of the dissociation temperature were observed in porous glasses, which had small pores ranging from 100 to 500 Angstrom in diameter, compared with that of the bulk hydrate at a given pressure. Systematic measurements revealed that the temperature offset was in inverse proportion to the pore diameter. The Arrhenius plot of the dissociation conditions suggests that the heat of methane-hydrate dissociation tended to be small compared to that of bulk hydrates in pores smaller than 300 A in diameter. Applying the Gibbs-Thomson effect to the quantitative analysis of the phenomenon indicated that the dissociation condition of methane hydrates in small pores shifted because of changes in the water activity. The apparent interfacial free energy between methane hydrates and water in the confined condition was estimated to be approximately 3.9 x 10(-2) J m(-2), which is comparable to that between ice and water in the similar condition.