A compound represented by formula (IA) or a pharmaceutically acceptable salt thereof, which is useful for the treatment or prevention of diseases such as sleep disorder, depression, anxiety disorders, panic disorder, schizophrenia, drug dependence, Alzheimer's disease, Parkinson's disease, Huntington's chorea, eating disorder, pain, digestive diseases, epilepsy, inflammations, immunity-related diseases, incretion-related diseases and hypertension, and of which the action relies on an orexin (OX) receptor antagonistic activity.一種以式(IA)所示之化合物或其醫藥上所容許之鹽，，其係可用於治療或預防基於食慾素(OX)受體拮抗作用所致之睡眠障礙、憂鬱症、焦慮症、恐慌障礙、統合失調症、藥物依賴症、阿茲海默症、帕金森氏症、杭丁頓[氏][舞蹈]症、攝食障礙、疼痛、消化器疾病、癲癇、炎症、免疫相關疾病、內分泌相關疾病、高血壓等疾病。
