A novel ion gel membrane composed of a specific inorganic/organic composite network and a large amount of room temperature ionic liquid (RTIL) was fabricated. Owing to the rigid and fragile inorganic component, the ion gel had very high mechanical strength. Even though the RTIL content was 80 wt%, the ion gel withstood more than 10 MPa of compressive stress. Owing to the high mechanical strength and good RTIL-holding property of the ion gel, the ion gel membrane had excellent pressure resistance, which could be used under pressurized conditions of more than 600 kPa for the trans-membrane pressure difference. Furthermore, it was confirmed that the ion gel membrane containing more than 70 wt% of 1-ethyl-3-methylimidazolium tetracyanoborate ([C(2)mim] [B(CN)(4)]) showed higher CO2 permeability than supported liquid membranes (SILMs). A comparison of the CO2 permeability of the composite ion gel membrane with 80 wt% of [C2mim][B(CN) 4] (ca. 2600 barrer) with the theoretically predicted maximum CO2 permeability of an [C(2)mim][B(CN)(4)]-based membrane (2910 barrer)indicated that the composite network showed a deficit of only 10% of the maximum CO2 permeability. The [C(2)mim][B(CN)(4)]-based inorganic/organic composite membrane showed better CO2 separation performancethan previously reported RTIL-based CO2 separation membranes and the obtained performances were beyond Robeson's upper bound.