In batch system for the catalytic decomposition of contaminants in wast water, the recovery of the catalytic particles is of great concern. One of the possible recovery means is to utilize magnetic separation. In the present work, in order to recover non-magnetic MnO2 catalytic particles, fine Fe3O4 particles were added into the solution and the coagulates were collected magnetically. The recovery efficiency was as high as 99%. Furthermore, the influence of Fe3O4 particles on the catalytic decomposition of H2O2 by MnO2 was investigated because H2O2 causes various problems in maintainning stable operation of wast water treatment system. The experimental results showed that the addition of Fe3O4 enhances the decomposition rate of H2O2 because MnO2 particles were well dispersed by Fe3O4. These results suggests that the introduction of fine Fe3O4 particles is effective for the decomposition of H2O2 as well as the recover of MnO2 catalytic particles.