To obtain an antibody production-inducing agent containing a tea ingredient having not been known as an active ingredient.

This antibody production-inducing agent contains a polysaccharide originating from tea as an active ingredient, and is a quite new antibody production-inducing agent. When the antibody production-inducing agent is orally taken, the antibody production-inducing agent can exhibit an excellent antibody production-inducing action, and is useful for preventing various infectious diseases caused by the deterioration of immune functions. This antibody production-inducing agent has high safety for human bodies and can continuously be taken over a long period, because a raw material for the antibody production-inducing agent is originated from a natural product having been supplied for eating uses.