WE CLAIM
1. A fresh water production method comprising treating a first raw water, wherein the first raw water is sewage, industrial wastewater, lake water, groundwater, river water or a treatment water obtained by subjecting sewage, industrial wastewater, lake water, groundwater or river water to pre- treatment other than in accordance with the fresh water production method, using a first semipermeable membrane treatment apparatus to produce fresh water and concentrated water, mixing the concentrated water produced by the treatment in the first semipermeable membrane treatment apparatus with a second raw water, wherein the second raw water is seawater, salt lake water, brine water or a treatment water obtained by subjecting seawater, salt lake water or brine water to pre-treatment other than in accordance with the fresh water production method and has an osmotic pressure higher than the osmotic pressure of the first raw water, to produce a mixed water, and treating the mixed water using a second semipermeable membrane treatment apparatus to produce fresh water, wherein a first microbicide is continuously or intermittently injected into the first raw water and a second microbicide is continuously or intermittently injected into the second raw water and wherein a third microbicide is continuously or intermittently injected into the concentrated water and, optionally, a fourth microbicide is continuously or intermittently injected into the mixed water.
2. The fresh water production method, as claimed in claim 1, wherein the concentration of the first microbicide in the concentrated water as discharged from the first semipermeable membrane treatment apparatus is larger than the concentration of the first microbicide in the first raw water as supplied to the first semipermeable membrane treatment apparatus.
3. The fresh water production method, as claimed in claim 1, wherein a first neutralizing agent with an effect of eliminating or decreasing a washing effect and a sterilization effect of the second microbicide is continuously or intermittently injected at a position between a) a means for injecting the second microbicide and b) a water mixing means for mixing the concentrated water and the second raw water.

4. The fresh water production method, as claimed in claim 3, wherein the first neutralizing agent has an effect of eliminating or decreasing a washing effect and a sterilization effect of the first microbicide.
5. The fresh water production method, as claimed in claim 4, wherein the first neutralizing agent is intermittently injected and the first microbicide is injected while the injection of the first neutralizing agent is suspended, and/or the third microbicide is injected while the injection of the first neutralizing agent is suspended, and/or the fourth microbicide is injected while the injection of the first neutralizing agent is suspended.

6. The fresh water production method, as claimed in claim1, wherein a first neutralizing agent with an effect of eliminating or decreasing the washing effect and a sterilization effect of at least one microbicide selected from the first, second and third microbicides is injected continuously or intermittently at a position between the water mixing means for mixing the concentrated water and the second raw water and a means for injecting the fourth microbicide.
7. The fresh water production method, as claimed in claim 6, wherein the second neutralizing agent and the fourth microbicide are intermittently injected, and the fourth microbicide is injected while the injection of the second neutralizing agent is suspended.
8. The fresh water production method, as claimed in claim 1, wherein in the case where the first microbicide is of a type identical to that of the at least one microbicide selected from the second, third and fourth microbicides, the at least one microbicide selected from the second, third and fourth microbicides is injected so that at least a portion of the microbicide selected from the second, third and fourth microbicides is mixed with the first microbicide.
9. The fresh water production method, as claimed in claim1, wherein in the case where a fourth microbicide is continuously or intermittently injected into the mixed water and the fourth microbicide is of a type identical to that of the at least one microbicide selected from the first, second and third microbicides, the at least one microbicide selected from the first, second and third microbicides is injected so that at least a portion of the microbicide selected from the first, second and third microbicide is mixed with the fourth microbicide .

10. A fresh water production method, as claimed in claim 1, wherein in the case where the first microbicide is of a type different from that of the second microbicide, the second microbicide is injected so that the second microbicide is not mixed with the first microbicide.
11. The fresh water production method, as claimed in claim1, wherein in the case where a fourth microbicide is continuously or intermittently injected into the mixed water and the fourth microbicide is of a type different from that of at least one microbicide selected from the first, second and third microbicides, the microbicide selected from the first, second and third microbicides, is injected so that the microbicide selected from the first, second and third microbicides is not mixed with the fourth microbicide.
12. The fresh water production method, as claimed in claim 9, wherein a first meter selected from the group consisting of a pH meter, ORP meter, chlorine concentration meter, EC meter, TOC meter, ammonia meter, TN meter and DO meter is installed between a water mixing means for mixing the concentrated water and the second raw water and a fourth microbicide injection means, wherein the injection amount of the fourth microbicide is determined on the basis of an indicated value of the first meter.
13. The fresh water production method, as claimed in claim 1, wherein in the case where at least one of the first microbicide or second microbicide is an acid or alkali, the ORP value of

the first raw water or second raw water into which the first microbicide or second microbicide is injected is adjusted to be kept within a specified range of values at a position upstream of an injection point of the first microbicide or second microbicide .
14. The fresh water production method, as claimed in claim 13, wherein in the case where at least one chemical selected from the first, second, third and fourth chemicals is an acid or alkali, a first ORP meter for measuring the ORP value of the liquid, a fifth chemical injection means for continuously or intermittently injecting an oxidizing agent or reducing agent as a fifth chemical to the liquid, and a second ORP meter for measuring the ORP value of the liquid containing the fifth chemical injected therein are installed in this order from the upstream side at positions upstream of all the injection means of the acid and alkali chemicals; the start of injecting the oxidizing agent or reducing agent by the fifth chemical injection means is determined in reference to the measured value of the first ORP meter; and the stop of the injection is determined in reference to the measured value of the second ORP meter.
15. The fresh water production method, as claimed in claim 1, wherein the ORP value of the concentrated water is measured, and if the ORP value is larger than a specified value, the concentrated water is discharged outside the equipment, and the semipermeable membrane treatment of the second semipermeable membrane treatment apparatus is stopped.

16. The fresh water production method, as claimed in claim 1, wherein the first semipermeable membrane treatment apparatus comprises chlorine-resistant semipermeable membranes and the first microbicide is a chlorine-based microbicide.
17. The fresh water production method, as claimed in claim 1, wherein the second semipermeable membrane treatment apparatus comprises chlorine-resistant semipermeable membranes and at least one of the first microbicide or second microbicide is a chlorine-based microbicide.
18. The fresh water production method, as claimed in claim 1, wherein the second microbicide is a chlorine-based microbicide, and nitrogen-containing water is supplied as the first raw water, and/or a nitrogen-containing microbicide is injected into the first raw water or the concentrated water or the mixed water.