A branched alkyl heteroaromatic ring derivative represented by the formula (Ia) or a pharmaceutically acceptable salt thereof is based on orexin (OX) receptor antagonism, sleep disorder, depression, anxiety disorder, panic disorder, integration Treatment of diseases such as ataxia, drug addiction, Alzheimer's disease, Parkinson's disease, Huntington's disease, eating disorders, headache, migraine, pain, digestive disorders, epilepsy, inflammation, immune related diseases, endocrine related diseases, hypertension Or it is useful for prevention.式（Ｉａ）で示される分岐鎖アルキルヘテロ芳香環誘導体、又はその医薬上許容される塩は、オレキシン（ＯＸ）受容体拮抗作用に基づいた、睡眠障害、うつ病、不安障害、パニック障害、統合失調症、薬物依存症、アルツハイマー病、パーキンソン病、ハンチントン舞踏病、摂食障害、頭痛、片頭痛、疼痛、消化器疾患、てんかん、炎症、免疫関連疾患、内分泌関連疾患、高血圧等の疾患の治療又は予防に有用である。
