A heteroaromatic methyl cyclic amine 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, Alzheimers disease, Parkinsons disease, Huntingtons 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.
