In an externally integrated steam generator type small modular reactor, a steam generator is arranged along the circumference of a reactor vessel cylindrical shell, and a steam drum is arranged along the circumference of the steam generator. 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 and including a first penetration hole communicating with an inside of the nuclear reactor; and the steam drum surrounding the circumference of the steam generator and including a second penetration hole communicating with an inside of the steam generator.