NOVELTY - A beta -glucosidase, comprising a beta -glucosidase catalytic domain which comprises (i) a polypeptide comprising a defined sequence of 874 amino acids (SEQ ID NO. 1), given in the specification, (ii) a polypeptide comprising an amino acid sequence in which one or more amino acids are deleted, substituted, or added in the amino acid sequence of SEQ ID NO. 1, and having beta -glucosidase activity, or (iii) a polypeptide comprising an amino acid sequence having 92% or greater sequence identity with the amino acid sequence of SEQ ID NO. 1, and having beta -glucosidase activity, is new.    USE - The beta -glucosidase is useful for producing a cellulose degradation product (claimed).    ADVANTAGE - Since, the beta -glucosidase has high hydrolase activity on cellulose, it is particularly preferable for enzymatic hydrolysis treatment of cellulose-based biomass.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are:    (1) a polynucleotide, comprising a region that encodes a beta -glucosidase catalytic domain which comprises (i) a base sequence that encodes a polypeptide comprising an amino acid sequence of SEQ ID NO. 1, (ii) a base sequence that encodes a polypeptide comprising an amino acid sequence in which one or more amino acids are deleted, substituted, or added in the amino acid sequence of SEQ ID NO. 1, and having beta -glucosidase activity, (iii) a base sequence that encodes a polypeptide comprising an amino acid sequence having 92% or greater sequence identity with the amino acid sequence of SEQ ID NO. 1, and having beta -glucosidase activity, or (iv) a base sequence of a polynucleotide which hybridizes with a polynucleotide comprising a defined nucleotide sequence of 2625 bp (SEQ ID NO. 2), given in the specification, under a stringent condition, and being a base sequence that encodes a polypeptide having beta -glucosidase activity;    (2) an expression vector, which is incorporated with the polynucleotide of (1), and which is able to express a polypeptide having beta -glucosidase activity in a host cell;    (3) a transformant which is introduced with the expression vector of (2);    (4) a method for producing a beta -glucosidase;    (5) a cellulase mixture comprising the beta -glucosidase above or a beta -glucosidase produced by the method of (4), and at least one type of other cellulases; and    (6) a method for producing a cellulose degradation product by generating a cellulose degradation product by contacting a cellulose-containing material with the beta -glucosidase above or a beta -glucosidase produced by the method of (4).