The present invention addresses the problem of providing signal sequence information that makes it possible for an antibody to be secreted extracellularly in antibody production by microorganisms of the genus Bifidobacterium and an antibody expression vector that makes it possible for an antibody to be secreted extracellularly using this signal sequence information. The present invention overcomes this problem by preparing Bifidobacterium longum transformed by a vector having a DNA insert, linked to the 5' terminus of the antibody gene, inserted at the 3' terminus of DNA encoding a signal peptide-linker linked body that has a linker linked to the C terminus of a signal peptide comprising an amino acid sequence shown in SEQ ID NO.: 1.
