A washing machine includes: a tub configured to accommodate wash water therein; a rotary tub rotatably mounted in the tub; a pulsator rotatably arranged below the rotary tub, configured to form a water current; a motor configured to provide driving power to the pulsator; and a floater configured to ascend or descend in response to a water level of the wash water in such a manner that the pulsator and the rotary tub interact with each other or the interaction therebetween is released. The pulsator includes a reinforcement rib protruding from a bottom surface of the pulsator to reinforce stiffness. The floater is connected to the reinforcement rib in a manner that the floater ascends or descends along the reinforcement rib. The washing machine can alternatively transmit the driving power of the motor only to the pulsator or to the pulsator and the rotary tub.