Dextransucrase forms a complex with dextran during an enzymatic reaction with sucrose. Using its enzymatic character, we performed a continuous and dynamic rejection of colloidal particles by generating dextran with dextransucrase immobilized in an inorganic porous membrane. Inorganic membranes having 1.9 and 3.0 U/g of immobilized dextransucrase, and 4.1 and 9.4 mg/g of generated dextran, respectively, had constant rejection percentages for 55 and 100 nm colloidal particles in permeating solutions. On the other hand, permeating sucrose solutions containing colloidal particles through a dextran-immobilized membrane dynamically increased the rejection percentages of the colloidal particles owing to dextran generation via enzymatic reaction. The dynamic increase was due to the gradually generating dextran dynamically occupying the membrane pore with its steric volume.