A branched chain alkyl heteroaromatic ring derivative represented by formula (Ia) or a pharmaceutically acceptable salt thereof is useful for treatment or prophylaxis of diseases such as sleep disorder, depression, anxiety disorder, panic disorder, schizophrenia, drug dependence, Alzheimer's disease, Parkinson's disease, Huntington's disease, eating disorder, cephalalgia, hemicrania, pain, digestive diseases, epilepsy, inflammation, immune-related diseases, endocrine-related diseases and hypertension, on the basis of an orexin (OX) receptor antagonist activity.一種以式(Ia)所示之支鏈烷基雜芳香環衍生物、或其醫藥上所容許之鹽，其係基於食慾激素(OX)受體之拮抗作用，來有效用於治療或預防睡眠障礙、憂鬱症、焦慮症、恐慌症、統合失調症、藥物依賴症、阿茲海默症、帕金森氏症、亨氏舞蹈症、飲食障礙、頭痛、偏頭痛、疼痛、消化器疾病、癲癇、炎症、免疫關連疾病、內分泌關連疾病、高血壓等疾病。
