Disclosed is a substance having an antagonistic effect on the binding of histamine to a histamine H3 receptor or an inhibitory effect on the activity which a histamine H3 receptor constantly exhibits. A compound represented by the formula (I) or a pharmaceutically acceptable salt thereof (I)wherein R1 represents a phenyl group which may be substituted or the like and R2 represents a cyano group which may be substituted or the like, or R1 and R2 together form an aliphatic heterocylic ring which may be substituted R3 represents a group represented by the formula (II-1) belowand all of X1 to X4 represent a carbon atom or the like: (II-1) where R4 and R5 represent a lower alkyl group or the like and ml represents an integer of 2 to 4.
