Morphinan derivative and medical usage thereof

The purpose of the present invention is to provide a compound that is effective as a prophylactic agent or a therapeutic agent for various diseases and symptoms associated with the orexin receptor, as well as to provide a medical usage thereof. The present invention provides a morphinan derivative that is represented by general formula (I), that exhibits superior selectivity and antagonism with respect to the orexin receptor, and that exhibits superior therapeutic and prophylactic effects with respect to drug dependency, or provides a pharmacologically acceptable acid-addition salt thereof, as well as an orexin-receptor antagonist and a drug-dependency therapeutic agent or prophylactic agent, that contain the morphinan derivative or the acid-addition salt as an active ingredient.