The present invention provides a novel treatment of alopecia areate. The problem to be solved by the invention is to provide a new pharmaceutical use of 3-[(3S, 4R)-3-methyl-6-(7H-pyrrolo[2, 3-d]pyrimidin-4-yl)-1, 6-diazaspiro[3.4]octan-1-yl]-3-oxopropanenitrile. A therapeutic or preventive agent for alopecia areata, containing 3-[(3S, 4R)-3-methyl-6-(7H-pyrrolo[2, 3-d]pyrimidin-4-yl)-1, 6-diazaspiro[3.4]octan-1-yl]-3-oxopropanenitrile as an active ingredient.