To prevent adhesion of condensate on a catalyst and enable efficiently recombining hydrogen and oxygen, by arranging the recombining catalyst in pipes connecting to gas phase in reactor pressure vessel without contacting the pipes.

In gas phase of instrumentation pipes 3 of main steam line, for example, of a boiling water reactor power station, catalyst 7 recombining hydrogen and oxygen generated by radiolysis of reactor water is placed by welding to isolatedly set from the inner wall. Therefore, gas layer 10 is provided between the catalyst 7 and the inner wall of pipe gas phase 3 and degradation of the catalyst 7 due to dewing can be avoided. Similar effect is obtained by placing the catalyst 7 with support legs. In this case, the support legs are welded to the pipe gas phase 3 and the catalyst 7, and the catalyst 7 is fixed in the pipe gas phase 3. For catalyst 7, that including noble metal elements such as platinum palladium, rhodium, ruthenium, iridium, osminium, etc., is effective.