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, Alzheimer&rsquos disease, Parkinson&rsquos disease, Huntington&rsquos 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.
