The present invention provides a novel diazepine compound that blocks the IKur current or the Kv1.5 channel potently and more selectively than other K+ channels. The present invention relates to a diazepine compound represented by General Formula (1) or a salt thereof, wherein R1, R2, R3, and R4 are each independently hydrogen, lower alkyl, cyclo lower alkyl or lower alkoxy lower alkyl; R2 and R3 may be linked to form lower alkylene; A1 is lower alkylene optionally substituted with one or more substituents selected from the group consisting of hydroxyl and oxo; Y1 and Y2 are each independently —N═ or —CH═; and R5 is group represented by wherein R6 and R7 are each independently hydrogen or organic group; R6 and R7 may be linked to form a ring together with the neighboring group —XA—N—XB—; XA and XB are each independently a bond, lower alkylene, etc.