The present invention relates to an apparatus for protecting a nuclear reactor building from a molten core generated from a nuclear reactor and, more specifically, to an insulator provided outside a nuclear reactor pressure vessel. A nuclear reactor insulator can comprise: an insulating body which is positioned at a predetermined distance away from the lateral side of a nuclear reactor pressure vessel and encircles the lateral side of the nuclear reactor pressure vessel, and of which the lower part is tapered; and a soluble plug which is provided on the bottom side of the insulating body and is for storing a molten core generated from the nuclear reactor pressure vessel, wherein the molten core generated from the lateral side of the nuclear reactor pressure vessel can be guided and discharged downward. The present invention can prevent thermal damage to a nuclear reactor building due to damage in the lateral side of the nuclear reactor pressure vessel, and enables installation of a core catcher of which the cross-sectional area has been reduced by means of guiding the molten core downward and increasing the predictability of the storage thereof. Moreover, the present invention enables reduction of a heat load in the core catcher by means of retarding the time of the molten core reaching the core catcher and reduces the risk of a steam explosion of the nuclear reactor building,due to multiple emission, by means of reducing the number of molten core emissions from the nuclear reactor pressure vessel to a nuclear reactor cavity.