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 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 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 . <EOS>