An apparatus and method for manufacturing a steel tube are provided. The apparatus includes: a tube-forming device for forming a steel plate into a steel tube; a heat treatment device connected in-line to the tube-forming device to heat the steel tube to a high temperature; a pre-treatment device for annealing the steel tube and providing a reduction atmosphere; and a plating device including a pot for storing a SeAHLume alloy composed of aluminum and zinc in a molten state, a level block selectively inserted into the molten alloy to adjust a level of the molten alloy, and a plating part into which the molten alloy flows in response to insertion of the level block and through which the steel tube passes substantially vertically. Therefore, it is possible to manufacture a steel tube having a plated surface to improve its corrosion-resistance.