Provided is a washing machine including a rib extending outward from a central portion of a rear portion of a tub. One end of the rib is provided adjacent to the central portion of the tub, the other end of the rib is provided adjacent to an outer side of the tub, and since a width of the one end of the rib is greater than a width of the other end of the rib, thereby maintaining efficient rigidity against stress generated in the rear portion of the tub. Further, in order to secure rigidity against stress due to vibrations generated by a driving motor, an additional rib is included between a side to which the driving motor is coupled and the outer side of the tub, thereby efficiently maintaining rigidity against additional stress.