An apparatus for manufacturing a steel pipe and a manufacturing method thereof are provided to improve corrosion resistance of the steel pipe by uniformly plating the aluminum-zinc alloy on the surface of the steel pipe and to easily control thickness of the alloy by injecting the inert gas to the steel pipe through the upper nozzle apparatus. An apparatus for manufacturing a steel pipe is composed of a pipe forming apparatus for forming a steel plate as a steel pipe(1); a heat-processing apparatus for heating-processing the steel pipe through the heating of hot temperature; a pre-treatment apparatus for slow-cooling and annealing the steel pipe; and a plating apparatus having a port(21) storing the welded corrosion resistant alloy containing 55 weight% aluminum and 43.4~44.9 weight% zinc, a level block(26) selectively inserted in the welded alloy, and a plating part(21a). Upper and lower guide rollers(31,30) are installed at the upper and lower ends of the steel pipe. An upper nozzle apparatus(34) is placed at the upper side of the plating part to control thickness of the alloy.