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.
