In a nuclear reactor building of a steel plate concrete structure that houses a pressure containment vessel formed with a plurality of penetration ports penetrating the pressure containment vessel on the periphery thereof and includes a biological shielding wall disposed outside the pressure containment vessel. The pressure containment vessel is vertically divided into a plurality of blocks so that each of the blocks has one or more pressure containment vessel penetration ports arranged on a same horizontal plane, and the reactor building including the biological shielding wall is divided into a plurality of modules by the horizontal plane.