Regarding the storage of fuel debris accompanying the decommissioning of the Fukushima Daiichi Nuclear Power Plant, management of hydrogen gas generated by radiolysis of water is indispensable. For this reason, it is important to maintain the concentration of hydrogen in sealed vessels below the explosion limit in order to improve the safety of long-term waste storage containers. Then, it was considered that the generated hydrogen is combined with oxygen by the passive autocatalytic recombiner (PAR) and return to water. Basic experiments and analyses were performed to investigate the controlling factors for reducing the hydrogen concentration in a fuel debris storage container with PAR. The present study shows the effects of the PAR quantity, PAR installation position and hydrogen flow rate on the reduction of hydrogen concentration.