An indole derivative compound represented by formula (I) (wherein the symbols in the formula are as mentioned in the specification) and a salt thereof Since the compounds represented by formula (I) binds to PGD.sub.2 receptors and shows antagonistic activity, they are believed to be useful for prevention and/or treatment of diseases such as allergic disease (such as allergic rhinitis, allergic conjunctivitis, atopic dermatitis, bronchial asthma and food allergy), systemic mastocytosis, systemic mast cell activating disorder, anaphylaxis shock, airway contraction, urticaria, eczema, diseases accompanied by itch (such as atopic dermatitis, urticaria), diseases (such as cataract, retinal detachment, inflammation, infection and sleep disorder) which are generated secondarily as a result of behavior accompanied by itch (such as scratching and beating), inflammation, chronic obstructive pulmonary diseases, ischemic reperfusion injury, cerebrovascular accident, autoimmune disease, chronic articular rheumatism, pleuritis, ulcerative colitis and irritable bowel syndrome.