[en] An electromotive valve is interposed to each of a plurality of depressurization pipelines branched from a main steam pipe, which is connected to a reactor pressure vessel. A rupture disk is disposed at the opening end of the depressurization pipeline at the downstream of the electromotive valve. A vent pipeline is disposed between the electromotive valve and the rupture disk, while being branched from the depressurization pipeline, for introducing leaked steams to a suppression pool. Then, a thermometer is disposed in the midway of the vent pipeline. If the electromotive valve is opened, the rupture disk is broken by high pressure steams flowing from the main steam pipe. With such procedures, steams in the reactor pressure vessel are released into the reactor container to depressurize the reactor pressure vessel. Further, the leakage of the electromotive valve during normal operation is easily detected by the thermometer disposed in the midway of the vent tube based on the temperature of the steams. (I.N.)