The present invention provides an oral vaccine against a bacterial infectious disease (e.g., typhoid fever, cholera, or dysentery). The oral vaccine of the present invention is a capsule formulation in which a transformed microorganism that expresses a flagellin antigen protein or that secretes a flagellin antigen protein out of the cell of the microorganism is encapsulated with an acid-resistant membrane. Examples of the microorganism include intestinal bacteria belonging to the genus Bifidobacterium, the genus Lactobacillus, the genus Lactococcus, and the like. The form of the capsule formulation may be any one of a seamless capsule formulation, a soft capsule formulation, and a hard capsule formulation.