Disclosed are a washing machine having a dual-drum of a main drum and a sub drum, and an assembly method thereof. The washing machine comprises: a cabinet; a tub provided in the cabinet; a main drum rotatably mounted in the tub; a sub drum mounted in the main drum so as to perform a relative rotation with the main drum; a hollow type outer shaft connected to the main drum; an inner shaft connected to the sub drum in the outer shaft; and a driving motor, wherein the driving motor comprises: a stator; an outer rotor connected to the inner shaft, and rotated outside the stator; and an inner rotor connected to the outer shaft, and rotated inside the stator, and wherein the driving motor drives the main drum and the sub drum in an independent manner.