A washing machine is disclosed. The washing machine include a first housing, a second housing arranged above the first housing, a first tub covered by the first housing, a second tub covered by the second housing, and a first detergent supplier arranged to supply a detergent to the first tub, a second detergent supplier having a first supply assembly arranged to supply a detergent to the second tub by siphonage when water for laundry is being supplied, wherein the first supply assembly comprises a first valve device arranged in a detergent supply path, in which a detergent is moved to the second tub, for preventing water for laundry from flowing backwards by buoyancy.