It is possible to eliminate fluctuation in the pellet strength of the entrapping immobilization pellets according to production lots, and constantly and stably produce the entrapping immobilization pellets having high pellet strength regardless of the production lots. An apparatus for producing entrapping immobilization pellets by polymerizing an immobilizing material into a gel in the presence of an activated sludge to entrap and immobilize microorganisms in the immobilizing material, the apparatus including: a line mixer which mixes an activated sludge with an immobilizing material to prepare a raw material solution, a temperature sensor which measures a temperature of the prepared raw material solution, an addition pump which adds a polymerization initiator to the prepared raw material solution, and a controller which controls the addition pump according to the measured temperature to control an additive rate of the polymerization initiator.