To assess the risk of a hydrogen recombination catalyst, its ignition characteristics were experimentally investigated. A layer of catalysts was installed perpendicular to a mixture flow of H-2 and air with controlled flow rates. The temperature on the catalyst and that of the mixture upstream of the catalyst were measured using thermocouples. The density variation of the mixture caused by combustion was observed using shadowgraph visualization. When both the flow velocity of the mixture and the fraction of H-2 were larger than the ignition thresholds, deflagration started and the temperature of the mixture upstream of the catalyst rapidly increased owing to the propagation of the flame front. The catalyst had the potential to ignite the mixture when the temperature of the catalyst, which increased with the amount of H-2 reacting on the catalyst, was sufficiently high. Therefore, it is necessary to maintain the flow velocity of the mixture and the fraction of H-2 below the thresholds while monitoring the surface temperature of the catalyst.