The present invention relates to a device for expanding a steel pipe and a method for manufacturing the expanded steel pipe by using the device for expanding the steel pipe and, more specifically, to a device for expanding a steel pipe in which manufacturing time and manufacturing costs are remarkably reduced and a method for manufacturing the expanded steel pipe by using the device for expanding the steel pipe. The device for expanding a steel pipe (1) according to an embodiment of the present invention comprises a settlement unit (10) having a plurality of settlement rollers (12); a dice (20) in which a locking protrusion (22) is formed; a transfer rail (30) whose each part is extended and both parts face each other in a horizontal direction from the front side of the dice (20); a transfer unit (40) having a wire (44); a transfer carriage (50); a tow member (60) installed in the transfer rail (30); a fixing member (70) installed in a block type; a mandrel (80) accommodated in an internal diameter of the steel pipe (1) to be moved by passing through a through-hole (21) according to the movement of the transfer bogie (50) as one end is fixed to a fixture (71); a plug (90) to precisely expand the internal diameter of the steel pipe (1).