An atopic dermatitis inducer binding to a human own IgE antibody and activating mast cells and basophiles, which includes a purified human secretion fraction, or an antigenic molecule or an antigenic determinant in the purified fraction, and obtained through the following steps of: filtering a human secretion, removing insoluble matters and collecting the filtrate; mixing the filtrate with a ConA-affinity carrier and collecting the supernatant; and separating a component having a histamine-releasing activity from the supernatant by column chromatography. This inducer is effective in diagnosing and treating human atopic dermatitis.