A therapeutic agent for allergic dermatitis caused by type IV allergic reaction, which contains diethyl L-lysylcromoglycate of the formula: ##STR1## or a nontoxic salt thereof as an active ingredient. This therapeutic agent for allergic dermatitis is effective for the treatment of allergic dermatitis caused by type IV allergic reaction, such as contact dermatitis mainly induced by type IV allergic reaction and atopic dermatitis induced by type I and type IV allergic reactions.