A washing machine comprises a first housing having a first laundry loading port, a first tub disposed in the first housing, a second housing having a second laundry loading port formed at a top portion and disposed on the first housing, a second tub disposed in the second housing, a water supply device connected to an external water source to supply wash water to the first tub and the second tub and installed at the second housing. The water supply device comprises a water supply connector having a plurality of chambers, a first water supply hose connecting the water supply connector with the first tub, a second water supply hose connecting the water supply connector with the second tub, and a backflow prevention structure formed in one of the plurality of the chambers to operate by a pressure of wash water supplied to the plurality of chambers.