The purpose of the present invention is to provide an antibody capable of inhibiting the infection of a cell with a norovirus. A nano antibody which contains a polypeptide as mentioned in (a), (b) or (c) and can inhibit the infection of an intestinal cell with HuNoV GII.3 and/or HuNoV GII.4: (a) a polypeptide comprising an amino acid sequence represented by SEQ ID NO: 1, SEQ ID NO: 5, SEQ ID NO: 9 or SEQ ID NO: 13; (b) a polypeptide comprising an amino acid sequence produced by substituting, deleting, inserting and/or adding one or several amino acid residues in an amino acid sequence represented by SEQ ID NO: 1, SEQ ID NO: 5, SEQ ID NO: 9 or SEQ ID NO: 13; and (c) a polypeptide comprising an amino acid sequence having a 80% or higher sequence identity to an amino acid sequence represented by SEQ ID NO: 1, SEQ ID NO: 5, SEQ ID NO: 9 or SEQ ID NO: 13.
