Disclosed are benzothiazine derivatives represented by the following formula (I): ##STR1## The benzothiazine derivatives (I) and their salts according to the present invention have strong serotonin-2 blocking action, have excellent selectivity to this action against .alpha..sub.1 blocking action and have high safety. Accordingly, the present invention has made it possible to provide pharmaceuticals making use of antagonistic action against serotonin-2 receptors, for example, therapeutics for various circulatory diseases such as ischemic heart diseases, cerebrovascular disturbances and peripheral circulatory disturbances.