A washing machine, comprising: a main drum rotatably mountable in a tub; a sub drum mounted inside 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; driving means configured to rotate the main drum and the sub drum in different directions and/or with different rotational speeds; a main drum spider to connect the main drum to the outer shaft to transfer a driving force of the driving means to the main drum, and a sub drum spider to connect the sub drum to the inner shaft to transfer a driving force of the driving means to the sub drum, wherein the two spiders (91, 95) are arranged to be independently rotatable, characterized in that: the sub drum has a front side open and a rear side closed by a sub drum back forming a rear surface, wherein the sub drum spider is disposed between the sub drum back and the main drum spider, and the sub drum back is provided with a receiving portion for receiving the sub drum spider, wherein the main drum spider comprises a shaft coupling portion coupled to the outer shaft, a spider supporting portion radially extending from the shaft coupling portion, and a drum fixing portion disposed at an end of the spider supporting portion for being fixed to the main drum, and wherein the drum fixing portion is formed in a ring shape for being coupled to an outer circumferential surface of the main drum. a main drum rotatably mountable in a tub; a sub drum mounted inside 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; driving means configured to rotate the main drum and the sub drum in different directions and/or with different rotational speeds; a main drum spider to connect the main drum to the outer shaft to transfer a driving force of the driving means to the main drum, and a sub drum spider to connect the sub drum to the inner shaft to transfer a driving force of the driving means to the sub drum, wherein the two spiders (91, 95) are arranged to be independently rotatable, characterized in that: the sub drum has a front side open and a rear side closed by a sub drum back forming a rear surface, wherein the sub drum spider is disposed between the sub drum back and the main drum spider, and the sub drum back is provided with a receiving portion for receiving the sub drum spider, wherein the main drum spider comprises a shaft coupling portion coupled to the outer shaft, a spider supporting portion radially extending from the shaft coupling portion, and a drum fixing portion disposed at an end of the spider supporting portion for being fixed to the main drum, and wherein the drum fixing portion is formed in a ring shape for being coupled to an outer circumferential surface of the main drum. the sub drum has a front side open and a rear side closed by a sub drum back forming a rear surface, wherein the sub drum spider is disposed between the sub drum back and the main drum spider, and the sub drum back is provided with a receiving portion for receiving the sub drum spider, wherein the main drum spider comprises a shaft coupling portion coupled to the outer shaft, a spider supporting portion radially extending from the shaft coupling portion, and a drum fixing portion disposed at an end of the spider supporting portion for being fixed to the main drum, and wherein the drum fixing portion is formed in a ring shape for being coupled to an outer circumferential surface of the main drum.