WE CLAIM:
1. A method of producing a protein using a fungus belonging to the genus Trichoderma, said method comprising culturing a fungus belonging to the genus Trichoderma whose BXL1 gene was disrupted, using a biomass containing cellulose and xylan as an inducer.
2. The method according to claim 1, wherein the culturing is carried out adding said inducer to the culture medium to a concentration of at least 5% by weight (wt/wt).
3. The method according to claim 1 or 2, wherein said inducer has a xylan content of 15 to 40% by weight.
4. The method according to any one of claims 1 to 3, wherein said fungus belonging to the genus Trichoderma is Trichoderma reesei.
5. The method according to claim 4, wherein said Trichoderma reesei is a strain in which carbon catabolite repression is removed.
6. The method according to any one of claims 1 to 5, wherein said protein is a cellulase composition.
7. The method according to claim 6, wherein p-xylosidase specific activity in said cellulase composition in terms of an enzyme activity for degrading p-nitrophenyl-p-D-xylopyranoside per 1 mg of protein in said cellulase composition is 0.006 U/mg protein or less; wherein cellobiohydrolase specific activity in terms of an enzyme activity for degrading/>-nitrophenyl-p-D-lactopyranoside per 1 mg of protein in said cellulase is at least 0.1 U/mg protein; and wherein (3-glucosidase specific activity in terms of an enzyme activity for degrading /?-nitrophenyl-p-D-glucopyranoside per 1 mg of protein in said cellulase is at least 0.25 U/mg protein.
8. The method according to claim 7, wherein said p-xylosidase specific activity is 0.002 U/mg protein or less; and wherein said p-glucosidase specific activity is at least 0.3 U/mg protein.
9. A method of producing xylo-oligosaccharides, said method comprising hydrolyzing a biomass containing xylan and cellulose with said cellulase composition produced by said method according to any one of claims 6 to 8.
10. A method of producing xylo-oligosaccharides and glucose, said method comprising hydrolyzing a biomass containing xylan and cellulose with said cellulase composition produced by said method according to any one of claims 6 to 8.

11. A method of suppressing decrease in dissolved oxygen saturation when culturing a fungus belonging to the genus Trichoderma using a biomass containing cellulose and xylan as an inducer, said method comprising using a fungus belonging to the genus Trichoderma whose BXL1 gene was disrupted as said fungus belonging to the genus Trichoderma.
12. A cellulase composition having the following characteristics (a) to (d):

(a) (3-xylosidase specific activity in said cellulase composition in terms of an enzyme activity for degrading ^>-nitrophenyl-(3-D-xylopyranoside per 1 mg of protein in said cellulase composition is 0.006 U/mg protein or less;
(b) cellobiohydrolase specific activity in terms of an enzyme activity for degrading/>-nitrophenyl-(3-D-lactopyranoside per 1 mg of protein in said cellulase composition is at least 0.1 U/mg protein;
(c) (3-glucosidase specific activity in terms of an enzyme activity for degrading p-nitrophenyl-P-D-glucopyranoside per 1 mg of protein in said cellulase composition is at least 0.25 U/mg protein; and
(d) the protein concentration of said cellulase composition is 3 g/L or more.

13. The cellulase composition according to claim 12, wherein said protein concentration is 9 g/L or more.
14. The cellulase composition according to claim 12 or 13, whereinp-nitrophenyl-p-D-xylobioside-degrading activity per 1 mg of protein in said cellulase composition is at least 5 U/mg protein.