Abstract: A recombination apparatus is provided to an off-gas system of a boiling water nuclear plant. An off-gas system pipe connected to a condenser is connected to the recombination apparatus. A catalyst layer filled with a catalyst for recombining hydrogen and oxygen is disposed in the recombination apparatus. The recombination catalyst has a percentage of the number of Pt particles whose diameters are in a range from more than 1 nm to not more than 3 nm to the numbers of Pt particles whose diameters are in a range from more than 0 nm to not more than 20 nm, falling within a range from 20 to 100%. The condenser discharges gas containing an organosilicon compound (ex. D5), hydrogen, and oxygen, which is introduced to the recombination apparatus. Use of the above recombination catalyst can improve the performance of recombining hydrogen and oxygen more than conventional catalysts and the initial performance of the catalyst can be maintained for a longer period of time.