Filter device

[Problem] To provide a filter device capable of stably filtering, at a fixed flow rate, a liquid to be treated. [Solution] This filter device comprises: one or a connected plurality of first units, each comprising a first separation membrane part that separates a liquid to be treated in a first storage tank into a first filtered liquid and a first non-filtered liquid, a first takeout tube that takes out the first non-filtered liquid, a first adjustment unit that keeps the flow rate of the first filtered liquid substantially constant, and a first liquid level meter that detects the liquid level in the first storage tank; a second unit comprising a second separation membrane that separates a storage liquid in a second storage tank in which the first filtered liquid is accumulated into a second filtered liquid and a second non-filtered liquid, a second takeout tube that takes out the second non-filtered liquid, a second adjustment unit that keeps the flow rate of the second filtered liquid substantially constant, and a second liquid level meter that detects the liquid level in the second storage tank; a first control unit that controls the liquid level in the first storage tank on the basis of a measurement value from two connected first units, or a measurement value from a connected first unit and second unit; and a second control unit that controls the liquid level in the second storage tank on the basis of a measurement value from the second unit.