The present invention provide a compound of the formula (I) wherein ring A represents an aromatic ring optionally having substituents; B, Y and Ya are the same or different and each represents a bond, etc.; R.sup.1 and R.sup.2 are the same or different and each represents a hydrogen atom, etc.; R.sup.3 represents a hydrogen atom, etc.; R.sup.4 and R.sup.5 are the same or different and each represents a hydrogen, etc.; R.sup.6 represents an indolyl group optionally having substituents; and Z and Za are the same or different and each represents a hydrogen atom, etc.; or a salt thereof or a prodrug thereof, having a somatostatin receptor binding inhibition activity and is useful for preventing and/or treating diseases associated with somatostatin.