A washing machine having a dual-drum, comprising: a cabinet which forms an appearance of the washing machine; a tub provided in the cabinet; a main drum rotatably mounted in the tub; a sub drum (60,60') mounted in the main drum to be relatively rotatable with respect to the main drum; an outer shaft for rotating the main drum; an inner shaft for rotating the sub drum (60,60'), being disposed inside the outer shaft; a main drum spider configured to connect the main drum and the outer shaft to each other; a sub drum spider (95,95') configured to connect the sub drum (60,60') and the inner shaft to each other; and a driving motor, wherein the driving motor comprises: a stator; an outer rotor connected to the inner shaft, and rotated outside the stator, characterized in that the driving motor further comprises: an inner rotor connected to the outer shaft, and rotated inside the stator, wherein the sub drum (60,60') comprises: a cylindrical portion (61,61') which forms an outer circumferential surface of the sub drum (60,60'); and a sub drum back (62,62') which forms a rear surface of the sub drum (60,60'), and to which the sub drum spider (95,95') is coupled, and wherein the sub drum back (62,62') comprises an accommodation portion (63,63') forward-concaved from the sub drum back (62,62') and configured to accommodate the sub drum spider (95,95') at a rear side thereof. a cabinet which forms an appearance of the washing machine; a tub provided in the cabinet; a main drum rotatably mounted in the tub; a sub drum (60,60') mounted in the main drum to be relatively rotatable with respect to the main drum; an outer shaft for rotating the main drum; an inner shaft for rotating the sub drum (60,60'), being disposed inside the outer shaft; a main drum spider configured to connect the main drum and the outer shaft to each other; a sub drum spider (95,95') configured to connect the sub drum (60,60') and the inner shaft to each other; and a driving motor, wherein the driving motor comprises: a stator; an outer rotor connected to the inner shaft, and rotated outside the stator, characterized in that the driving motor further comprises: an inner rotor connected to the outer shaft, and rotated inside the stator, wherein the sub drum (60,60') comprises: a cylindrical portion (61,61') which forms an outer circumferential surface of the sub drum (60,60'); and a sub drum back (62,62') which forms a rear surface of the sub drum (60,60'), and to which the sub drum spider (95,95') is coupled, and wherein the sub drum back (62,62') comprises an accommodation portion (63,63') forward-concaved from the sub drum back (62,62') and configured to accommodate the sub drum spider (95,95') at a rear side thereof. a stator; an outer rotor connected to the inner shaft, and rotated outside the stator, characterized in that the driving motor further comprises: an inner rotor connected to the outer shaft, and rotated inside the stator, wherein the sub drum (60,60') comprises: a cylindrical portion (61,61') which forms an outer circumferential surface of the sub drum (60,60'); and a sub drum back (62,62') which forms a rear surface of the sub drum (60,60'), and to which the sub drum spider (95,95') is coupled, and wherein the sub drum back (62,62') comprises an accommodation portion (63,63') forward-concaved from the sub drum back (62,62') and configured to accommodate the sub drum spider (95,95') at a rear side thereof. an inner rotor connected to the outer shaft, and rotated inside the stator, wherein the sub drum (60,60') comprises: a cylindrical portion (61,61') which forms an outer circumferential surface of the sub drum (60,60'); and a sub drum back (62,62') which forms a rear surface of the sub drum (60,60'), and to which the sub drum spider (95,95') is coupled, and wherein the sub drum back (62,62') comprises an accommodation portion (63,63') forward-concaved from the sub drum back (62,62') and configured to accommodate the sub drum spider (95,95') at a rear side thereof. a cylindrical portion (61,61') which forms an outer circumferential surface of the sub drum (60,60'); and a sub drum back (62,62') which forms a rear surface of the sub drum (60,60'), and to which the sub drum spider (95,95') is coupled, and wherein the sub drum back (62,62') comprises an accommodation portion (63,63') forward-concaved from the sub drum back (62,62') and configured to accommodate the sub drum spider (95,95') at a rear side thereof.