The present invention provides dendritic cells (DCs) having reduced inflammatory activity, a method for producing DCs capable of inducing regulatory T cells and regulatory T cells, and substances useful for such a method, wherein the DCs and regulatory T cells are useful for prevention and treatment of immune diseases. The present invention relates to a cereal plant-derived material fermented with a koji microorganism, which is obtained by fermentation thereof with a koji microorganism, or a processed product thereof. Preferably, the fermented product or a processed product thereof comprises 14-dehydroergosterol. According to the present invention, regulatory DCs and regulatory T cells can be induced and the proliferation of inflammatory T cells can be suppressed using plant-derived materials.