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.