A novel silica-pillared manganese oxide (SiHMnO) was synthesized, and its porous structure was exactly determined by assuming a multistep adsorption mechanism (MSAM) using the low-temperature N-2, Ar, and CO adsorption and the alpha(s)-comparison plot based on a standard N-2 adsorption isotherm on a nonporous manganese oxide. The MSAM method gave a good agreement in values of micropore volume (0.155-0.160 mL g(-1)) and external surface area (61-63 m(2).9(-1)) from the three adsorbates. The standard nonporous manganese oxide was selected by comparing those based on crystalline hydrous birnessite (H-Bir). The H-Bir-393 obtained by calcining H-Bir at 393 K is the most appropriate standard nonporous material. The values of micropore volume and external surface area from the a.-plot matched very well with those from the MSAM method. SiHMnO has a very high surface area value (483 m(2).g(-1)) by analysis of an alpha(s)-plot in contrast to the smaller one (360 m(2).g(-1)) by the ordinary BET analysis.