An object is to provide a signal peptide that can secrete a heterologous polypeptide with high efficiency outside the bacterial cell in a bacterium of the genus Bifidobacterium, an expression cassette that can secrete a heterologous polypeptide with high efficiency outside the bacterial cell, a heterologous polypeptide expression vector, a bacterium of the genus Bifidobacterium capable of secreting a heterologous polypeptide. Means for attaining the object is a bacterium of the genus Bifidobacterium transformed by a vector having an expression cassette sequentially comprising a promoter DNA functioning in a bacterium of the genus Bifidobacterium, a DNA encoding the secretory signal peptide, a DNA encoding a scFv antibody having an antitumor activity, and a terminator DNA functioning in the bacterium of the genus Bifidobacterium； and capable of secreting the scFv antibody with high efficiency outside the bacterial cell.
