A mutant xylanase comprising the amino acid sequence of SEQ ID NO: 2 with an amino acid residue substitution at position 154 in which the leucine residue is substituted by methionine and the following additional amino acid residue substitutions, or: the isoleucine residue at position 30 is substituted by valine; the asparagine residue at position 33 is substituted by aspartic acid; and the glycine residue at position 36 is substituted by arginine; the asparagine residue at position 33 is substituted by aspartic acid; the glycine residue at position 36 is substituted by arginine; the threonine residue at position 90 is substituted by serine; the glutamine residue at position 132 is substituted by arginine; the serine residue at position 174 is substituted by threonine; the proline residue at position 195 is substituted by histidine; the serine residue at position 197 is substituted by asparagine; and the glycine residue at position 217 is substituted by glutamic acid, the isoleucine residue at position 30 is substituted by valine; the serine residue at position 59 is substituted by threonine; the tyrosine residue at position 239 is substituted by histidine; and the cysteine residue at position 242 is substituted by serine or an amino acid sequence that has an identity of 90% or higher with at least one of: (i) the amino acid sequence of SEQ ID NO: 2 with the amino acid residue substitution at position 154 and the additional amino acid residue substitutions; (ii) the amino acid sequence of SEQ ID NO: 2 with the amino acid residue substitution at position 154 and the additional amino acid residue substitutions, or; (iii) the amino acid sequence of SEQ ID NO: 2 with the amino acid residue substitution at position 154 and the additional amino acid residue substitutions, and that includes the amino acid residue substitution at position 154 of SEQ ID NO: 2 and at least one selected from the group consisting of: (a) the additional amino acid residue substitutions, (b) the additional amino acid residue substitutions, and (c) the additional amino acid residue substitutions; wherein said mutant xylanase is a thermostable xylanase providing an initial rate of reaction that is at least 70% of that provided by SEQ ID No: 2 and having a xylanase activity after heat treatment at 50 °C for 24 hours that is at least 50% of its xylanase activity before the heat treatment as determined following the measurement method set forth in the description,. the isoleucine residue at position 30 is substituted by valine; the asparagine residue at position 33 is substituted by aspartic acid; and the glycine residue at position 36 is substituted by arginine; the asparagine residue at position 33 is substituted by aspartic acid; the glycine residue at position 36 is substituted by arginine; the threonine residue at position 90 is substituted by serine; the glutamine residue at position 132 is substituted by arginine; the serine residue at position 174 is substituted by threonine; the proline residue at position 195 is substituted by histidine; the serine residue at position 197 is substituted by asparagine; and the glycine residue at position 217 is substituted by glutamic acid, the isoleucine residue at position 30 is substituted by valine; the serine residue at position 59 is substituted by threonine; the tyrosine residue at position 239 is substituted by histidine; and the cysteine residue at position 242 is substituted by serine (i) the amino acid sequence of SEQ ID NO: 2 with the amino acid residue substitution at position 154 and the additional amino acid residue substitutions; (ii) the amino acid sequence of SEQ ID NO: 2 with the amino acid residue substitution at position 154 and the additional amino acid residue substitutions, or; (iii) the amino acid sequence of SEQ ID NO: 2 with the amino acid residue substitution at position 154 and the additional amino acid residue substitutions, and that includes the amino acid residue substitution at position 154 of SEQ ID NO: 2 and at least one selected from the group consisting of: (a) the additional amino acid residue substitutions, (b) the additional amino acid residue substitutions, and (c) the additional amino acid residue substitutions; (a) the additional amino acid residue substitutions, (b) the additional amino acid residue substitutions, and (c) the additional amino acid residue substitutions;