A passive catalytic reactor without heating is required to enhance the safety of a fusion facility. A precious metal catalyst without heating is not suitable to oxidize tritium under conditions of low hydrogen concentration and room temperature. In addition, under a moisture condition, tritium oxidation of a precious metal catalyst drops drastically since moisture adsorbs active sites on the surface of the catalyst. Hence, as a method of tritium oxidation under a moisture condition at room temperature, we have focused on bacterial oxidation of tritium by hydrogen-oxidizing bacteria in natural soil to realize a passive reactor. In this study, we investigated the effect of hydrogen concentration on tritium oxidation by hydrogen-oxidizing bacteria in natural soils to understand the characteristic of tritium oxidation by hydrogen-oxidizing bacteria from the viewpoint of engineering. In our experiment, efficiency of tritium oxidation by a natural soil was obtained at room temperature in the range of hydrogen concentration from 0.5 to 10 000 parts per million (ppm) under a moisture condition. The efficiency of tritium oxidation was the highest at a hydrogen concentration of 0.5 ppm, which equals the value of the hydrogen concentration in air. Our results show that hydrogen-oxidizing bacteria could efficiently oxidize tritium with a low concentration of hydrogen, at room temperature, with high moisture. This showed a tendency opposite to a metal catalyst. A bioreactor using hydrogen-oxidizing bacteria complemented a conventional catalytic reactor using a precious metal catalyst since hydrogen-oxidizing bacteria could oxidize tritium efficiently with a low concentration of hydrogen, at room temperature, with high moisture.