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: 3 or SEQ ID NO: 11; (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: 3 or SEQ ID NO: 11, and having hydrolytic activity using carboxymethyl cellulose as a substrate at least under conditions of a temperature of 110° C. and a pH of 4.0; 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: 3 or SEQ ID NO: 11, and having hydrolytic activity using carboxymethyl cellulose as a substrate at least under conditions of a temperature of 110° C. and a pH of 4.0.