Provided is a viscosity reducer for a mannan-containing material, whereby the utility of mannan can be enhanced. A viscosity reducer for a mannan-containing material, including a polypeptide provided with an additional sequence in which two or more histidines occur in succession at the C terminus of an amino acid sequence selected from the group comprising (a) through (f). (a): A polypeptide having an amino acid sequence represented by SEQ ID NO: 2. (b): A polypeptide having an amino acid sequence at least 70% identical to the amino acid sequence represented by SEQ ID NO: 2. (c) A polypeptide having an amino acid sequence in which one or a plurality of amino acids are substituted, deleted, and/or inserted in the amino acid sequence represented by SEQ ID NO: 2. (d): A polypeptide encoded by DNA which hybridizes under stringent conditions with DNA comprising the base sequence for encoding the amino acid sequence represented by SEQ ID NO: 2, or the base sequence complementary thereto. (e): A polypeptide encoded by a base sequence at least 70% identical to the base sequence represented by SEQ ID NO: 1. (f): A polypeptide encoded by DNA which hybridizes under stringent conditions with DNA comprising the base sequence for encoding the amino acid sequence represented by SEQ ID NO: 2, or the base sequence complementary thereto.L'invention concerne un réducteur de viscosité pour une substance contenant du mannane, moyennant quoi l'utilité du mannane peut être accrue. Le réducteur de viscosité pour une substance contenant du mannane comprend un polypeptide pourvu d'une séquence supplémentaire dans laquelle au moins deux histidines peuvent apparaître à la suite à l'extrémité C d'une séquence d'acides aminés choisie dans le groupe comprenant les polypeptides (a) à (f). (a) : Un polypeptide ayant une séquence d'acides aminés représentée par SEQ ID N° : 2. (b) : Un polypeptide ayant une séquence d'acides aminés identique à hauteur d'au moins 70 % à la séquence d'acides aminés rep
