To obtain an agent for treating dermatitis having high suppressing effects on the tendency to an increase in histamine and providing antipruritic effects on atopic dermatitis, etc.

This agent is used by application to a lesion, consists essentially of a water-alcohol mixture and further contains an ingredient extracted from Ganoderma lucidum Karst. as an active ingredient. At this time, the solid concentration of the active ingredient in the agent is preferably ≥0.5 wt.% and the alcohol concentration is preferably ≥10%. An antlerlike Ganoderma lucidum Karst. is preferred as the Ganoderma lucidum Karst. When the active ingredient is extracted from the Ganoderma lucidum Karst., a water-alcohol-based solvent is usually used. An alcohol alone or an organic solvent such as ethyl acetate, however, can be used. When the agent is sprayed on an inflamed site, the agent has effects on stop of pruritus and prevention of occurrence of inflammation by scratching.