It is an object of the present invention to provide signal sequence information capable of secreting an antibody to the outside of cells in generation of the antibody by microorganisms of genus Bifidobacterium, and an antibody expression vector capable of secreting an antibody to the outside of cells by utilizing the signal sequence information. As a means for achieving the aforementioned object, there is prepared Bifidobacterium longum, which is transformed with a vector having inserted thereinto a DNA insert comprising the 5′-terminus of an antibody gene linked to the 3′-terminus of a DNA encoding a signal peptide-linker conjugate having a linker linked to the C-terminus of a signal peptide consisting of an amino acid sequence shown in SEQ ID NO: 1.