A nuclear power plant according to an embodiment comprises: a reactor well; a reactor well upper lid; an operation floor; an operation floor area wall; a standby gas treatment system; and a reactor well exhaust section to release the gas inside the reactor well to the environment without releasing the gas into the operation floor area in an event of a predetermined accident, e.g., causing diminished cooling of a containment vessel or otherwise increasing its temperature. The standby gas treatment system includes: a suction pipe to take in gas inside the reactor building; an exhaust fan; a standby gas treatment system exhaust pipe; a heater that is disposed between the suction pipe and the standby gas treatment system exhaust pipe; and a filter to filter the gas heated by the heater and to send the gas to the standby gas treatment system exhaust pipe.