The present invention addresses the problem of providing: a signal peptide which enables the highly efficient secretion of a heterogeneous polypeptide produced in a cell of a bacterium belonging to the genus Bifidobacterium to the outside of the cell； an expression cassette which enables the efficient secretion of the heterogeneous polypeptide to the outside of the cell； an expression vector for the heterogeneous polypeptide； and a bacterium belonging to the genus Bifidobacterium, in which the heterogeneous polypeptide can be secreted. A means for solving the problem is a bacterium belonging to the genus Bifidobacterium, which is transformed with a vector having an expression cassette inserted thereinto, wherein the expression cassette comprises promoter DNA capable of acting in a bacterium belonging to the genus Bifidobacterium, DNA encoding the secretion signal peptide, DNA encoding a single-strand antibody having an anti-tumor activity, and terminator DNA capable of acting in a bacterium belonging to the genus Bifidobacterium in this order. The single-strand antibody produced in the Bifidobacterium bacterium can be secreted efficiently to the outside of a cell of the bacterium.
