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.
