A fermentation product (fermented food) that has high functionality by subjecting a fructan-containing material (in particular, a material including garlic or rakkyo containing fructan at a high concentration) directly to lactic acid fermentation without performing a heat treatment or an enzymatic treatment. More specifically, provided are: a fermentation product having an immunopotentiating effect, which is obtained by lactic acid fermentation of a fructan-containing material using a lactic acid bacterium Lactobacillus plantarum S506 strain (NITE BP-643) having fructan-utilizing ability, a mutant strain of the S506 strain having fructan-utilizing ability, or a strain isolated from Lactobacillus plantarum and having the same bacteriological properties as those of the S506 strain; and a fermented food which contains the fermentation product.