Provided are a treatment device enabling a remarkable reduction in the footprint and the power cost and also enabling an increase in the durability of movable members and a treatment bag used for the treatment device. The treatment device is characterized in that the inside of a water tank is partitioned into the inner space of the body of the treatment bag and the outer space thereof in which filling water is stored by immersing the treatment bag in the water tank in which the filling water always kept at a specified level is stored. The treatment bag comprises the body sealingly formed of a water impermeable and flexible sheet and a supply port for supplying a liquid to be treated into the body and a discharge port for discharging the treated liquid to the outside.