In an externally integrated once-through steam generator type small modular reactor, a steam generator is arranged along the circumference of a reactor vessel, and secondary cooling water flows in heat transfer tubes and changes to superheated steam. The small modular reactor includes: a nuclear reactor including a hemispherical upper head, the reactor vessel cylindrical shell coupled to the upper head and extending downward from the upper head in a cylindrical shape, and a hemispherical lower head provided on a lower portion of the reactor vessel cylindrical shell, wherein a core is placed in the nuclear reactor; the steam generator surrounding all around the reactor vessel cylindrical shell, the steam generator including a first penetration hole communicating with an inside of the nuclear reactor and a second penetration hole separate from the first penetration hole and communicating with the inside of the nuclear reactor.