A washing machine includes a first washing tub, and a second washing tub detachably coupled to the first washing tub. The second washing tub includes a container containing laundry, a lower cover coupled to a top of the container and having an inlet configured to receive a water stream moving upwards in the container when the first washing tub rotates, and an upper cover coupled to a top of the lower cover and having an opening for putting the laundry into the container. A flow path is provided between the lower cover and the upper cover to discharge the water stream introduced through the inlet to an outside of the second washing tub. A slide lock is provided on the upper cover, movable between a binding position and an unbinding position, bound to the lower cover in the binding position, and separated from the lower cover in the unbinding position.