For the practical use of activated carbon (AC) as an adsorbent of CH4, tightly packed monoliths with high microporosity are supposed to be one of the best morphologies in terms of storage capacity per apparent volume of the adsorbent material. However, monolith-type ACs may cause diffusion obstacles in adsorption processes owing to their necked pore structures among the densely packed particles, which result in a lower adsorption performance than that of the corresponding powder ACs. To clarify the relationship between the pore structure and CH4 adsorptivity, microscopic observations, structural studies on the nanoscale, and conductivity measurements (thermal and electrical) were performed on recently developed binder-free, self-sinterable ACs in both powder and monolithic forms. The monolith samples exhibited higher surface areas and electrical conductivities than the corresponding powder samples. Supercritical CH4 adsorption isotherms were measured for each powder and monolith sample at up to 7 MPa at 263, 273, and 303 K to elucidate their isosteric heats of adsorption and adsorption rate constants, which revealed that the morphologies of the monolith samples did not cause serious drawbacks for the adsorption and desorption processes. This will further facilitate the availability of diffusion-barrier-free microporous carbon monoliths as practical CH4 storage adsorbents.