The present invention provides a protolytically leaving group-containing copolymer having improved characteristics such as increased solubility in alkaline aqueous solution in the presence of a proton, thus finding application with advantage in various uses, and a process for producing the copolymer. 5 nm on the surface of a lower electrode of the capacitor. The catalyst absorbs NOx when the air-fuel ratio in the surrounding atmosphere thereof is lean and releases the absorbed NOx to purify NOx by reduction when the air-fuel ratio is stoichiometric or rich. A first hydraulic drive is disposed to assist the clamping force of the clamping spring, and a second hydraulic drive is disposed to oppose the clamping force of the clamping spring.