A novel soluble well-defined ladder polysiloxane bridged by imido linkages having a high molecular weight was successfully synthesized via the corresponding supramolecular intermediate and was fabricated to a very flexible self-standing membrane. The membrane showed a better oxygen permselectivity than conventional linear polyorganosiloxanes. We directly observed the effect of the regular molecular-sized pores formed by the ladder structure on the oxygen permselectivity for the first time. This is the first report on gas permeability thorough a well-characterized ladder polymer.