A reactor pressure vessel depressurization system for reducing pressure in a nuclear reactor facility, the system comprising: a reactor pressure vessel contained in a reactor containment vessel and a main steam safety relief valve arranged in the reactor containment vessel to discharge steam in the reactor pressure vessel into the reactor containment vessel in an abnormal condition of the nuclear reactor facility; a main steam safety relief valve driving gas pipe connected at a first end thereof to the main steam safety relief valve to lead first driving gas to the main steam safety relief valve; a three way electromagnetic valve arranged in the reactor containment vessel so as to be connected at a first connection port thereof to a second end of the main steam safety relief valve driving gas pipe opposite to the first end; a driving gas feed pipe connected to a second connection port of the three way electromagnetic valve to supply first drive gas from a supply source thereof; and a containment vessel external connection pipe connected at a third end thereof to a third connection port of the three way electromagnetic valve and extending to outside of the reactor containment vessel by way of a penetration section of the reactor containment vessel; wherein the three way electromagnetic valve is switchably either in a first communication state in which the first connection port does not communicate with the third connection port but communicates with the second connection port or in a second communication state in which the first connection port does not communicate with the second connection port but communicates with the third connection port, and is in the second communication state when the nuclear reactor facility is in normal operation; the containment vessel external connection pipe has: an open communication section formed either at inside or at the outside of the reactor containment vessel so as to be open in normal operation of the nuclear reactor facility and be capable of being shifted into a closed state, and an external gas receiving section capable of receiving second driving gas from a pressurization source outside of the reactor containment vessel, and the main steam safety relief valve can be opened by second driving gas supplied from the pressurization source in a state where the open communication section is not opened and in the second communication state. a reactor pressure vessel contained in a reactor containment vessel and a main steam safety relief valve arranged in the reactor containment vessel to discharge steam in the reactor pressure vessel into the reactor containment vessel in an abnormal condition of the nuclear reactor facility; a main steam safety relief valve driving gas pipe connected at a first end thereof to the main steam safety relief valve to lead first driving gas to the main steam safety relief valve; a three way electromagnetic valve arranged in the reactor containment vessel so as to be connected at a first connection port thereof to a second end of the main steam safety relief valve driving gas pipe opposite to the first end; a driving gas feed pipe connected to a second connection port of the three way electromagnetic valve to supply first drive gas from a supply source thereof; and a containment vessel external connection pipe connected at a third end thereof to a third connection port of the three way electromagnetic valve and extending to outside of the reactor containment vessel by way of a penetration section of the reactor containment vessel; wherein the three way electromagnetic valve is switchably either in a first communication state in which the first connection port does not communicate with the third connection port but communicates with the second connection port or in a second communication state in which the first connection port does not communicate with the second connection port but communicates with the third connection port, and is in the second communication state when the nuclear reactor facility is in normal operation; the containment vessel external connection pipe has: an open communication section formed either at inside or at the outside of the reactor containment vessel so as to be open in normal operation of the nuclear reactor facility and be capable of being shifted into a closed state, and an external gas receiving section capable of receiving second driving gas from a pressurization source outside of the reactor containment vessel, and an open communication section formed either at inside or at the outside of the reactor containment vessel so as to be open in normal operation of the nuclear reactor facility and be capable of being shifted into a closed state, and an external gas receiving section capable of receiving second driving gas from a pressurization source outside of the reactor containment vessel, and the main steam safety relief valve can be opened by second driving gas supplied from the pressurization source in a state where the open communication section is not opened and in the second communication state.