A hyperthermostable endoglucanase including an endoglucanase catalytic domain, the endoglucanase catalytic domain including: (A) a polypeptide including an amino acid sequence represented by SEQ ID NO: 1; (B) a polypeptide including an amino acid sequence in which at least one amino acid is deleted, substituted, or added in the amino acid sequence represented by SEQ ID NO: 1, and having hydrolytic activity using p-nitrophenyl-β-D-cellobioside as a substrate at least under conditions of a temperature of 100° C. and a pH of 5.5; or (C) a polypeptide including an amino acid sequence having at least 70% sequence identity with the amino acid sequence represented by SEQ ID NO: 1, and having hydrolytic activity using p-nitrophenyl-β-D-cellobioside as a substrate at least under conditions of a temperature of 100° C. and a pH of 5.5.