WE CLAIM:
1. A method of producing a sugar liquid, the method
comprising:
the step of obtaining an alkaline filtrate by allowing an alkaline aqueous medium to pass through a cellulose-containing biomass;
the recirculation filtration step of allowing the alkaline filtrate to repeatedly pass through the cellulose-containing biomass to produce a cellulose-containing solid component; and
the step of hydrolyzing the cellulose-containing solid component to obtain the sugar liquid.
2. The method of producing a sugar liquid according to claim 1, wherein the step of obtaining the alkaline filtrate comprises supplying the cellulose-containing biomass and the alkaline aqueous medium to a filtration apparatus, and allowing the alkaline aqueous medium to pass through the cellulose-containing biomass using the filtration apparatus.
3. The method of producing a sugar liquid according to claim 1 or 2, wherein the alkaline aqueous medium or the alkaline filtrate is allowed to pass through the cellulose-containing biomass by self-weight filtration in the direction of gravity.
4. The method of producing a sugar liquid according to any one of claims 1 to 3, wherein the alkaline aqueous medium and the alkaline filtrate are maintained substantially at the same temperature.
5. The method of producing a sugar liquid according to any one of claims 1 to 4, wherein the alkaline aqueous medium and the alkaline filtrate have a temperature of 80°C or higher and 100°C or lower.

6. The method of producing a sugar liquid according to any one of claims 1 to 5, wherein the alkaline filtrate comprises acetic acid or a salt thereof.
7. The method of producing a sugar liquid according to any one of claims 1 to 6, wherein the cellulose-containing biomass is one which has been sifted through a sieve with an aperture of 30 mm or more.
8. The method of producing a sugar liquid according to any one of claims 1 to 7, wherein the cellulose-containing biomass is one which has been subjected to a dry grinding treatment.
9. The method of producing a sugar liquid according to any one of claims 1 to 8, wherein the cellulose-containing biomass is an herbaceous biomass.
10. The method of producing a sugar liquid according to any one of claims 1 to 9, wherein the alkaline aqueous medium and the alkaline filtrate comprise at least one hydroxide selected from sodium hydroxide and potassium hydroxide.
11. The method of producing a sugar liquid according to any one of claims 1 to 10, wherein the alkaline aqueous medium and the alkaline filtrate are alkaline aqueous solutions.
12. The method of producing a sugar liquid according to any one of claims 1 to 11, wherein the period of time during which the alkaline filtrate is repeatedly passed through the cellulose-containing biomass is 30 minutes or more and three hours or less.
13. The method of producing a sugar liquid according to any one of claims 1 to 12, wherein the alkaline filtrate at the completion of the recirculation filtration step has a pH of 10 or more and 12 or less.

14. The method of producing a sugar liquid according to any one of claims 1 to 13, wherein the hydrolysis is carried out using an enzyme.
15. A pretreatment method of a cellulose-containing biomass, for hydrolyzing the cellulose-containing biomass to obtain a sugar liquid, the method comprising:
the step of obtaining an alkaline filtrate by allowing an alkaline aqueous medium to pass through a cellulose-containing biomass; and
the recirculation filtration step of allowing the alkaline filtrate to repeatedly pass through the cellulose-containing biomass to produce a cellulose-containing solid component.
16. A method of producing a cellulose-containing solid
component, wherein the cellulose-containing solid component is
obtained by the pretreatment method according to claim 15,
from the cellulose-containing biomass.