NOVELTY - A polypeptide having crystalline cellulose binding activity, is new.    USE - The polypeptide is useful for enhancing cellulose decomposition activity of cellulases useful for decomposing cellulose containing material. The decomposed product of cellulose containing material is useful for producing a useful substance (all claimed). The cellulose containing material is chosen from agricultural waste products such as straw, regenerated fiber goods, natural fiber goods (cotton and hemp), rayon, acetate fiber, and lyocell fiber, rice straw, construction waste material and lignocellulose. The decomposed product of cellulose containing material is used as biomass for producing biofuel.    ADVANTAGE - The polypeptide is highly efficient, and reduces the usage amount of cellulases for decomposition.    DETAILED DESCRIPTION - A polypeptide having crystalline cellulose binding activity, is new. The polypeptide is chosen from (a) polypeptide (P1) comprising a 356 amino acid sequence (SEQ ID NO: 2) fully defined in the specification, (b) polypeptide (P2) comprising an amino acid sequence capable of exhibiting 70% or more homology with an amino acid sequence of SEQ ID NO: 2, (c) polypeptide (P3) comprising an amino acid sequence of SEQ ID NO: 2, in which one or more amino acid residues are inserted, deleted and/or substituted, (d) polypeptide (P4) encoded by a DNA capable of hybridizing under stringent conditions with a DNA comprising a base pair sequence capable of encoding the polypeptide (P1), or its complementary base pair sequence, (e) polypeptide (P4) encoded by a DNA comprising a base pair sequence exhibiting at least 70% or more homology with the base pair sequence comprising a 1071 base pair sequence (SEQ ID NO: 1) fully defined in the specification, and (f) polypeptide (P6) encoded by a DNA capable of hybridizing under stringent conditions with a DNA comprising a base pair sequence of SEQ ID NO: 1, or its complementary base pair sequence. INDEPENDENT CLAIMS are included for the following:    (1) DNA construct comprising a polynucleotide capable of encoding the above-mentioned polypeptide and an element for its expression;    (2) expression vector comprising the above-mentioned DNA sequence;    (3) transformed cell comprising the above-mentioned DNA sequence;    (4) method (M1) for producing the above-mentioned polypeptide, involves culturing the above-mentioned transformed cell and recovering the polypeptide from the cultured cell;    (5) method (M2) for producing decomposed product of cellulose containing material, involves decomposing the cellulose containing material using the above-mentioned polypeptide, and one or more cellulases;    (6) method (M3) for producing a substance from cellulose containing material, involves (a) decomposing the cellulose containing material using the above-mentioned polypeptide, and one or more cellulases and (b) culturing a fermentation microorganism using the obtained decomposed product of cellulose containing material as a carbon source; and    (7) cellulase composition comprising the above-mentioned polypeptide and one or more cellulases.