The compound represented by formula (IA) or a pharmaceutically acceptable salt thereof is based on orexin (OX)receptor antagonist activity, is useful in the treatment and prevention of illnesses including sleep disorder, depression, anxiety disorder, panic disorder, schizophrenia, drug dependency, Alzheimer's disease, Parkinson's disease, Huntington's chorea, eating disorder, pain, gastrointestinal disease, epilepsy, inflammation, immunological disease, endocrinological related disease, and hypertension.
