1. A filtration device comprising:
a single or a plurality of continuous first membrane filtration units including a first storage tank configured to store therein a liquid to be treated, a first separation membrane part configured to separate the liquid to be treated into a first permeated liquid and a first non-permeated liquid, a first liquid delivery pipe configured to deliver the liquid to be treated from the first storage tank to the first separation membrane part, a first non-permeated liquid pipe configured to return the first non-permeated liquid to the first storage tank or the first liquid delivery pipe, a first permeated liquid pipe configured to cause the first permeated liquid to flow from the first separation membrane part, a first extraction pipe through which a portion of the first non-permeated liquid is extracted, a first adjustment unit configured to adjust a flow rate of the first permeated liquid to be substantially constant, and a first liquid scale configured to detect an amount of a stored liquid in the first storage tank;
a second membrane filtration unit including a second storage tank configured to store therein the first permeated liquid from the first permeated liquid pipe of the single first membrane filtration unit or a last-stage one of the first membrane filtration units, a second separation membrane part configured to separate a stored liquid in the second storage tank into a second permeated liquid and a second non-permeated liquid, a second liquid delivery pipe configured to deliver the stored liquid in the second storage tank from the second storage tank to the second separation membrane part, a second non-permeated liquid pipe configured to return the second non-permeated liquid to the second storage tank or the second liquid delivery pipe, a second permeated liquid pipe configured to cause the second permeated liquid to flow from the second separation membrane part, a second extraction pipe through which a portion of the second non-permeated liquid is extracted, a second adjustment unit configured to adjust a flow rate of the second permeated liquid to be substantially constant, and a second liquid scale configured to detect an amount of the

stored liquid in the second storage tank;
a first controller unit configured to control an amount of a stored liquid stored in the first storage tank based on measurement values from the first liquid scales of continuous two of the first membrane filtration units or measurement values from the first and second liquid scales of the continuous first and second membrane filtration units; and
a second controller unit configured to control an amount of a stored liquid stored in the second storage tank based on a measurement value from the second liquid scale.
2. The filtration device according to claim 1, wherein, based on a preceding-stage measurement value from a first liquid scale of a preceding first membrane filtration unit of the continuous two first membrane filtration units or the continuous first and second membrane filtration units and a subsequent-stage measurement value from a first or second liquid scale in a subsequent first or second membrane filtration unit, the first controller unit stops a flow of the first permeated liquid of the preceding first membrane filtration unit.
3. The filtration device according to claim 2, wherein when the preceding-stage measurement value is smaller than a first threshold or when the subsequent-stage measurement value is larger than a second threshold, the first controller unit adjusts the first adjustment unit of the preceding first membrane filtration unit to stop a flow of the first permeated liquid from the first permeated liquid pipe to the subsequent first or second membrane filtration unit.
4. The filtration device according to claim 2, wherein
the first membrane filtration unit further includes a first open/close valve provided in the first permeated liquid pipe, and
when the preceding-stage measurement value is smaller than a first threshold or when the subsequent-stage measurement value is larger than a second threshold, the first controller unit

closes the first open/close valve of the preceding first membrane filtration unit to stop a flow of the first permeated liquid from the first permeated liquid pipe to the subsequent first or second membrane filtration unit.
5. The filtration device according to claim 2, wherein
the first membrane filtration unit further includes a first permeated liquid returning pipe configured to return the first permeated liquid from the first permeated liquid pipe to the first storage tank, and a first non-permeated liquid returning pipe configured to return the first non-permeated liquid from the first extraction pipe to the first storage tank, and
when the preceding-stage measurement value is smaller than a first threshold or when the subsequent-stage measurement value is larger than a second threshold, the first controller unit returns the first permeated liquid in the first permeated liquid pipe to the first storage tank through the first permeated liquid returning pipe and returns the first non-permeated liquid in the first extraction pipe to the first storage tank through the first non-permeated liquid returning pipe in the preceding first membrane filtration unit.
6. The filtration device according to claim 1 or 2, wherein, based on a subsequent-stage measurement value from a second liquid scale in a subsequent second membrane filtration unit of the continuous first and second membrane filtration units, the second controller unit stops a flow of the second permeated liquid of the subsequent second membrane filtration unit.
7. The filtration device according to claim 6, wherein when the subsequent-stage measurement value of the subsequent second membrane filtration unit is smaller than a third threshold, the second controller unit adjusts the second adjustment unit of the subsequent second membrane filtration unit to stop a flow of the second permeated liquid from the second permeated liquid pipe.

8. The filtration device according to claim 6, further
comprising a second open/close valve provided in the second
permeated liquid pipe, wherein
when the subsequent-stage measurement value of the subsequent second membrane filtration unit is smaller than a third threshold, the second controller unit closes the second open/close valve of the subsequent second membrane filtration unit to stop a flow of the second permeated liquid from the second permeated liquid pipe.
9. The filtration device according to claim 6, wherein
the second membrane filtration unit further includes a second permeated liquid returning pipe configured to return the second permeated liquid from the second permeated liquid pipe to the second storage tank and a second non-permeated liquid returning pipe configured to return the second non-permeated liquid from the second extraction pipe to the second storage tank, and
when the subsequent-stage measurement value of the subsequent second membrane filtration unit is smaller than a third threshold, the second controller unit returns the second permeated liquid in the second permeated liquid pipe to the second storage tank through the second permeated liquid returning pipe, and returns the second non-permeated liquid in the second extraction pipe to the second storage tank through the second non-permeated liquid returning pipe.
10. The filtration device according to any one of claims 1 to 9,
wherein
a first permeated liquid from a preceding one of the continuous first membrane filtration units is stored in a first storage tank of a subsequent one of the continuous first membrane filtration units, and
the first controller unit of each of the first membrane filtration units controls an amount of a stored liquid stored in the first storage tank of the corresponding first membrane filtration

unit.
11. The filtration device according to any one of claims 1 to 10,
wherein
5 a pore diameter of a first separation membrane part of one
of the continuous first membrane filtration units, which is on a downstream side in a flow of the liquid to be treated, is smaller than a pore diameter of a first separation membrane part of the first membrane filtration unit on an upstream side, and
D a pore diameter of a second separation membrane part of
the second membrane filtration unit of the continuous first and second membrane filtration units, which is on a downstream side in a flow of the liquid to be treated, is smaller than a pore diameter of a first separation membrane part of the first membrane
5 filtration unit on an upstream side.
12. The filtration device according to any one of claims 1 to 11,
wherein each of the first and second separation membrane parts
is any of a microfiltration membrane, an ultrafiltration membrane,
D a nanofiltration membrane, and a reverse osmosis membrane.
13. The filtration device according to any one of claims 1 to 12,
further comprising a cleaning part connected to the first or second
permeated liquid pipe and configured to cause a cleaning solution
5 to flow from the first or second permeated liquid pipe to the first or second separation membrane part.
14. The filtration device according to any one of claims 1 to 13,
wherein the first or second liquid scale is a liquid level meter that
D detects a liquid level of a stored liquid in the first or second storage tank, or a mass measurement device that detects a mass of a stored liquid in the first or second storage tank.
15. The filtration device according to any one of claims 1 to 14,
5 wherein the liquid to be treated contains organic matter.

16. The filtration device according to any one of claims 1 to 15,
further comprising:
a third membrane filtration unit including a third storage tank configured to store therein the first non-permeated liquid from the first extraction pipe of the single first membrane filtration unit or any of the continuous first membrane filtration units, a third separation membrane part configured to separate a stored liquid in the third storage tank into a third permeated liquid and a third non-permeated liquid, a third liquid delivery pipe configured to deliver a stored liquid in the third storage tank from the third storage tank to the third separation membrane part, a third non-permeated liquid pipe configured to return the third non-permeated liquid to the third storage tank or the third liquid delivery pipe, a third permeated liquid pipe configured to deliver the third permeated liquid from the third separation membrane part to the first or second storage tank of the first or second membrane filtration unit in a subsequent stage, a third extraction pipe through which a portion of the third non-permeated liquid is extracted, a third adjustment unit configured to adjust a flow rate of the third permeated liquid to be substantially constant, and a third liquid scale configured to detect an amount of the stored liquid in the third storage tank; and
a third controller unit configured to control an amount of a stored liquid stored in the third storage tank based on measurement values from a third liquid scale of the third membrane filtration unit and the first or second liquid scale of the first or second membrane filtration unit in the subsequent stage.
17. The filtration device according to any one of claims 1 to 15,
further comprising:
a third membrane filtration unit including a third storage tank configured to store therein the second non-permeated liquid from the second extraction pipe, a third separation membrane part configured to separate a stored liquid in the third storage tank into a third permeated liquid and a third non-permeated liquid, a third liquid delivery pipe configured to deliver a stored

liquid in the third storage tank from the third storage tank to the third separation membrane part, a third non-permeated liquid pipe configured to return the third non-permeated liquid to the third storage tank or the third liquid delivery pipe, a third permeated liquid pipe configured to deliver the third permeated liquid from the third separation membrane part, a third extraction pipe through which a portion of the third non-permeated liquid is extracted, a third adjustment unit configured to adjust a flow rate of the third permeated liquid to be substantially constant, and a third liquid scale configured to detect an amount of a stored liquid in the third storage tank; and
a third controller unit configured to control an amount of a stored liquid stored in the third storage tank based on a measurement value from the third liquid scale of the third membrane filtration unit.
18. The filtration device according to any one of claims 1 to 17, wherein the first membrane filtration unit further includes a first water-adding part configured to add water to the liquid to be treated.
19. The filtration device according to any one of claims 1 to 18, wherein the second membrane filtration unit further includes a second water-adding part configured to add water to the first permeated liquid.
20. The filtration device according to any one of claims 16 to 19,
wherein the third membrane filtration unit further includes a third
water-adding part configured to add water to the first or second
non-permeated liquid.
21. A sugar solution manufacturing method comprising:
in the filtration device according to any one of claims 1 to 20,
a process of filtrating a liquid to be treated that contains sugar; and

a process of recovering a sugar solution as a non-permeated liquid or a permeated liquid in the filtrating.