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, and a sub drum mounted in the main drum to be relatively rotatable with respect to the main drum. The washing machine includes 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 and rotatable outside the stator, and an inner rotor connected to the outer shaft and rotatable inside the stator. The washing machine also includes a spring washer inserted into a connection part of the outer shaft and the inner rotor that attenuates vibrations of the outer shaft to prevent noise and to prevent separation of the outer shaft due to the vibrations.