A washing machine includes a main body defining an outer appearance, a tub disposed within the main body, a main drum rotatably mounted in the tub, a sub drum 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, being disposed inside the outer shaft, and a driving motor having a stator, an outer rotor connected to the inner shaft that is rotatable outside the stator, and an inner rotor connected to the outer shaft that is rotatable inside the stator, wherein the driving motor independently drives the main drum and the sub drum.