This letter reports the preparation and the performance of a novel microgel system in which an enzyme (urease) is physically entrapped in the lightly cross-linked polymer network having thermosensitive and ionic moieties. The preparation can easily be performed by aqueous redox polymerization in the presence of both surfactant and enzyme. The microgel obtained undergoes a rapid change in diameter from 215 to 155 nm as an immobilized enzyme reaction sets in. When the gel particles are loaded into the pores of a cellulose membrane, we can enzymatically open the gate pore among the particles to allow the passage of a liquid through the membrane. These results strongly suggest that the enzyme immobilization method based on thermosensitive polyelectrolyte microgels has a large spectrum of technical applications.