This invention provides a compound or its pharmaceutically-acceptable salt of formula (I) wherein R.sup.1 represents a lower alkyl group et al; R.sup.2 and R.sup.3 are same and different and represents hydrogen atm et al; R.sup.4 represents the substituent of the formula (I) et al; X.sub.1 represents NH, O or S; Y represents N or C; Ar is a divalent substituent derived from aryl et al, by removing two hydrogen atoms therefrom; the ring A represents a 5- or 6-membered heteroaryl group; this compounds has a histamine-H3 receptor antagonistic effect or a histamine-H3 receptor inverse-agonistic effect and is useful for preventive or remedy of metabolic system diseases, circulatory system diseases or nervous system diseases.